diff --git a/langchain/chains/__init__.py b/langchain/chains/__init__.py
index e5f8b7b23b..90e0b9225a 100644
--- a/langchain/chains/__init__.py
+++ b/langchain/chains/__init__.py
@@ -4,6 +4,7 @@ from langchain.chains.api.openapi.chain import OpenAPIEndpointChain
 from langchain.chains.combine_documents.base import AnalyzeDocumentChain
 from langchain.chains.combine_documents.map_reduce import MapReduceDocumentsChain
 from langchain.chains.combine_documents.map_rerank import MapRerankDocumentsChain
+from langchain.chains.combine_documents.reduce import ReduceDocumentsChain
 from langchain.chains.combine_documents.refine import RefineDocumentsChain
 from langchain.chains.combine_documents.stuff import StuffDocumentsChain
 from langchain.chains.constitutional_ai.base import ConstitutionalChain
@@ -111,4 +112,5 @@ __all__ = [
     "MapRerankDocumentsChain",
     "MapReduceDocumentsChain",
     "RefineDocumentsChain",
+    "ReduceDocumentsChain",
 ]
diff --git a/langchain/chains/combine_documents/base.py b/langchain/chains/combine_documents/base.py
index 92e7838ff6..0693aac44a 100644
--- a/langchain/chains/combine_documents/base.py
+++ b/langchain/chains/combine_documents/base.py
@@ -11,30 +11,20 @@ from langchain.callbacks.manager import (
 )
 from langchain.chains.base import Chain
 from langchain.docstore.document import Document
-from langchain.schema import BasePromptTemplate
 from langchain.text_splitter import RecursiveCharacterTextSplitter, TextSplitter
 
 
-def format_document(doc: Document, prompt: BasePromptTemplate) -> str:
-    """Format a document into a string based on a prompt template."""
-    base_info = {"page_content": doc.page_content}
-    base_info.update(doc.metadata)
-    missing_metadata = set(prompt.input_variables).difference(base_info)
-    if len(missing_metadata) > 0:
-        required_metadata = [
-            iv for iv in prompt.input_variables if iv != "page_content"
-        ]
-        raise ValueError(
-            f"Document prompt requires documents to have metadata variables: "
-            f"{required_metadata}. Received document with missing metadata: "
-            f"{list(missing_metadata)}."
-        )
-    document_info = {k: base_info[k] for k in prompt.input_variables}
-    return prompt.format(**document_info)
-
-
 class BaseCombineDocumentsChain(Chain, ABC):
-    """Base interface for chains combining documents."""
+    """Base interface for chains combining documents.
+
+    Subclasses of this chain deal with combining documents in a variety of
+    ways. This base class exists to add some uniformity in the interface these types
+    of chains should expose. Namely, they expect an input key related to the documents
+    to use (default `input_documents`), and then also expose a method to calculate
+    the length of a prompt from documents (useful for outside callers to use to
+    determine whether it's safe to pass a list of documents into this chain or whether
+    that will longer than the context length).
+    """
 
     input_key: str = "input_documents"  #: :meta private:
     output_key: str = "output_text"  #: :meta private:
@@ -58,25 +48,57 @@ class BaseCombineDocumentsChain(Chain, ABC):
     def prompt_length(self, docs: List[Document], **kwargs: Any) -> Optional[int]:
         """Return the prompt length given the documents passed in.
 
-        Returns None if the method does not depend on the prompt length.
+        This can be used by a caller to determine whether passing in a list
+        of documents would exceed a certain prompt length. This useful when
+        trying to ensure that the size of a prompt remains below a certain
+        context limit.
+
+        Args:
+            docs: List[Document], a list of documents to use to calculate the
+                total prompt length.
+
+        Returns:
+            Returns None if the method does not depend on the prompt length,
+            otherwise the length of the prompt in tokens.
         """
         return None
 
     @abstractmethod
     def combine_docs(self, docs: List[Document], **kwargs: Any) -> Tuple[str, dict]:
-        """Combine documents into a single string."""
+        """Combine documents into a single string.
+
+        Args:
+            docs: List[Document], the documents to combine
+            **kwargs: Other parameters to use in combining documents, often
+                other inputs to the prompt.
+
+        Returns:
+            The first element returned is the single string output. The second
+            element returned is a dictionary of other keys to return.
+        """
 
     @abstractmethod
     async def acombine_docs(
         self, docs: List[Document], **kwargs: Any
     ) -> Tuple[str, dict]:
-        """Combine documents into a single string asynchronously."""
+        """Combine documents into a single string.
+
+        Args:
+            docs: List[Document], the documents to combine
+            **kwargs: Other parameters to use in combining documents, often
+                other inputs to the prompt.
+
+        Returns:
+            The first element returned is the single string output. The second
+            element returned is a dictionary of other keys to return.
+        """
 
     def _call(
         self,
         inputs: Dict[str, List[Document]],
         run_manager: Optional[CallbackManagerForChainRun] = None,
     ) -> Dict[str, str]:
+        """Prepare inputs, call combine docs, prepare outputs."""
         _run_manager = run_manager or CallbackManagerForChainRun.get_noop_manager()
         docs = inputs[self.input_key]
         # Other keys are assumed to be needed for LLM prediction
@@ -92,6 +114,7 @@ class BaseCombineDocumentsChain(Chain, ABC):
         inputs: Dict[str, List[Document]],
         run_manager: Optional[AsyncCallbackManagerForChainRun] = None,
     ) -> Dict[str, str]:
+        """Prepare inputs, call combine docs, prepare outputs."""
         _run_manager = run_manager or AsyncCallbackManagerForChainRun.get_noop_manager()
         docs = inputs[self.input_key]
         # Other keys are assumed to be needed for LLM prediction
@@ -104,7 +127,12 @@ class BaseCombineDocumentsChain(Chain, ABC):
 
 
 class AnalyzeDocumentChain(Chain):
-    """Chain that splits documents, then analyzes it in pieces."""
+    """Chain that splits documents, then analyzes it in pieces.
+
+    This chain is parameterized by a TextSplitter and a CombineDocumentsChain.
+    This chain takes a single document as input, and then splits it up into chunks
+    and then passes those chucks to the CombineDocumentsChain.
+    """
 
     input_key: str = "input_document"  #: :meta private:
     text_splitter: TextSplitter = Field(default_factory=RecursiveCharacterTextSplitter)
@@ -131,6 +159,7 @@ class AnalyzeDocumentChain(Chain):
         inputs: Dict[str, str],
         run_manager: Optional[CallbackManagerForChainRun] = None,
     ) -> Dict[str, str]:
+        """Split document into chunks and pass to CombineDocumentsChain."""
         _run_manager = run_manager or CallbackManagerForChainRun.get_noop_manager()
         document = inputs[self.input_key]
         docs = self.text_splitter.create_documents([document])
diff --git a/langchain/chains/combine_documents/map_reduce.py b/langchain/chains/combine_documents/map_reduce.py
index 84e49296a5..6fc30415eb 100644
--- a/langchain/chains/combine_documents/map_reduce.py
+++ b/langchain/chains/combine_documents/map_reduce.py
@@ -2,74 +2,97 @@
 
 from __future__ import annotations
 
-from typing import Any, Callable, Dict, List, Optional, Protocol, Tuple
+from typing import Any, Dict, List, Tuple
 
 from pydantic import Extra, root_validator
 
 from langchain.callbacks.manager import Callbacks
 from langchain.chains.combine_documents.base import BaseCombineDocumentsChain
+from langchain.chains.combine_documents.reduce import ReduceDocumentsChain
 from langchain.chains.llm import LLMChain
 from langchain.docstore.document import Document
 
 
-class CombineDocsProtocol(Protocol):
-    """Interface for the combine_docs method."""
+class MapReduceDocumentsChain(BaseCombineDocumentsChain):
+    """Combining documents by mapping a chain over them, then combining results.
 
-    def __call__(self, docs: List[Document], **kwargs: Any) -> str:
-        """Interface for the combine_docs method."""
+    We first call `llm_chain` on each document individually, passing in the
+    `page_content` and any other kwargs. This is the `map` step.
 
+    We then process the results of that `map` step in a `reduce` step. This should
+    likely be a ReduceDocumentsChain.
 
-def _split_list_of_docs(
-    docs: List[Document], length_func: Callable, token_max: int, **kwargs: Any
-) -> List[List[Document]]:
-    new_result_doc_list = []
-    _sub_result_docs = []
-    for doc in docs:
-        _sub_result_docs.append(doc)
-        _num_tokens = length_func(_sub_result_docs, **kwargs)
-        if _num_tokens > token_max:
-            if len(_sub_result_docs) == 1:
-                raise ValueError(
-                    "A single document was longer than the context length,"
-                    " we cannot handle this."
-                )
-            if len(_sub_result_docs) == 2:
-                raise ValueError(
-                    "A single document was so long it could not be combined "
-                    "with another document, we cannot handle this."
-                )
-            new_result_doc_list.append(_sub_result_docs[:-1])
-            _sub_result_docs = _sub_result_docs[-1:]
-    new_result_doc_list.append(_sub_result_docs)
-    return new_result_doc_list
-
-
-def _collapse_docs(
-    docs: List[Document],
-    combine_document_func: CombineDocsProtocol,
-    **kwargs: Any,
-) -> Document:
-    result = combine_document_func(docs, **kwargs)
-    combined_metadata = {k: str(v) for k, v in docs[0].metadata.items()}
-    for doc in docs[1:]:
-        for k, v in doc.metadata.items():
-            if k in combined_metadata:
-                combined_metadata[k] += f", {v}"
-            else:
-                combined_metadata[k] = str(v)
-    return Document(page_content=result, metadata=combined_metadata)
+    Example:
+        .. code-block:: python
 
+            from langchain.chains import (
+                StuffDocumentsChain,
+                LLMChain,
+                ReduceDocumentsChain,
+                MapReduceDocumentsChain,
+            )
+            from langchain.prompts import PromptTemplate
+            from langchain.llms import OpenAI
 
-class MapReduceDocumentsChain(BaseCombineDocumentsChain):
-    """Combining documents by mapping a chain over them, then combining results."""
+            # This controls how each document will be formatted. Specifically,
+            # it will be passed to `format_document` - see that function for more
+            # details.
+            document_prompt = PromptTemplate(
+                input_variables=["page_content"],
+                 template="{page_content}"
+            )
+            document_variable_name = "context"
+            llm = OpenAI()
+            # The prompt here should take as an input variable the
+            # `document_variable_name`
+            prompt = PromptTemplate.from_template(
+                "Summarize this content: {context}"
+            )
+            llm_chain = LLMChain(llm=llm, prompt=prompt)
+            # We now define how to combine these summaries
+            reduce_prompt = PromptTemplate.from_template(
+                "Combine these summaries: {context}"
+            )
+            reduce_llm_chain = LLMChain(llm=llm, prompt=reduce_prompt)
+            combine_documents_chain = StuffDocumentsChain(
+                llm_chain=reduce_llm_chain,
+                document_prompt=document_prompt,
+                document_variable_name=document_variable_name
+            )
+            reduce_documents_chain = ReduceDocumentsChain(
+                combine_documents_chain=combine_documents_chain,
+            )
+            chain = MapReduceDocumentsChain(
+                llm_chain=llm_chain,
+                reduce_documents_chain=reduce_documents_chain,
+            )
+            # If we wanted to, we could also pass in collapse_documents_chain
+            # which is specifically aimed at collapsing documents BEFORE
+            # the final call.
+            prompt = PromptTemplate.from_template(
+                "Collapse this content: {context}"
+            )
+            llm_chain = LLMChain(llm=llm, prompt=prompt)
+            collapse_documents_chain = StuffDocumentsChain(
+                llm_chain=llm_chain,
+                document_prompt=document_prompt,
+                document_variable_name=document_variable_name
+            )
+            reduce_documents_chain = ReduceDocumentsChain(
+                combine_documents_chain=combine_documents_chain,
+                collapse_documents_chain=collapse_documents_chain,
+            )
+            chain = MapReduceDocumentsChain(
+                llm_chain=llm_chain,
+                reduce_documents_chain=reduce_documents_chain,
+            )
+    """
 
     llm_chain: LLMChain
     """Chain to apply to each document individually."""
-    combine_document_chain: BaseCombineDocumentsChain
-    """Chain to use to combine results of applying llm_chain to documents."""
-    collapse_document_chain: Optional[BaseCombineDocumentsChain] = None
-    """Chain to use to collapse intermediary results if needed.
-    If None, will use the combine_document_chain."""
+    reduce_documents_chain: BaseCombineDocumentsChain
+    """Chain to use to reduce the results of applying `llm_chain` to each doc.
+    This typically either a ReduceDocumentChain or StuffDocumentChain."""
     document_variable_name: str
     """The variable name in the llm_chain to put the documents in.
     If only one variable in the llm_chain, this need not be provided."""
@@ -93,6 +116,29 @@ class MapReduceDocumentsChain(BaseCombineDocumentsChain):
         extra = Extra.forbid
         arbitrary_types_allowed = True
 
+    @root_validator(pre=True)
+    def get_reduce_chain(cls, values: Dict) -> Dict:
+        """For backwards compatibility."""
+        if "combine_document_chain" in values:
+            if "reduce_documents_chain" in values:
+                raise ValueError(
+                    "Both `reduce_documents_chain` and `combine_document_chain` "
+                    "cannot be provided at the same time. `combine_document_chain` "
+                    "is deprecated, please only provide `reduce_documents_chain`"
+                )
+            combine_chain = values["combine_document_chain"]
+            collapse_chain = values.get("collapse_document_chain")
+            reduce_chain = ReduceDocumentsChain(
+                combine_documents_chain=combine_chain,
+                collapse_documents_chain=collapse_chain,
+            )
+            values["reduce_documents_chain"] = reduce_chain
+            del values["combine_document_chain"]
+            if "collapse_document_chain" in values:
+                del values["collapse_document_chain"]
+
+        return values
+
     @root_validator(pre=True)
     def get_return_intermediate_steps(cls, values: Dict) -> Dict:
         """For backwards compatibility."""
@@ -123,11 +169,31 @@ class MapReduceDocumentsChain(BaseCombineDocumentsChain):
         return values
 
     @property
-    def _collapse_chain(self) -> BaseCombineDocumentsChain:
-        if self.collapse_document_chain is not None:
-            return self.collapse_document_chain
+    def collapse_document_chain(self) -> BaseCombineDocumentsChain:
+        """Kept for backward compatibility."""
+        if isinstance(self.reduce_documents_chain, ReduceDocumentsChain):
+            if self.reduce_documents_chain.collapse_documents_chain:
+                return self.reduce_documents_chain.collapse_documents_chain
+            else:
+                return self.reduce_documents_chain.combine_documents_chain
+        else:
+            raise ValueError(
+                f"`reduce_documents_chain` is of type "
+                f"{type(self.reduce_documents_chain)} so it does not have "
+                f"this attribute."
+            )
+
+    @property
+    def combine_document_chain(self) -> BaseCombineDocumentsChain:
+        """Kept for backward compatibility."""
+        if isinstance(self.reduce_documents_chain, ReduceDocumentsChain):
+            return self.reduce_documents_chain.combine_documents_chain
         else:
-            return self.combine_document_chain
+            raise ValueError(
+                f"`reduce_documents_chain` is of type "
+                f"{type(self.reduce_documents_chain)} so it does not have "
+                f"this attribute."
+            )
 
     def combine_docs(
         self,
@@ -141,14 +207,24 @@ class MapReduceDocumentsChain(BaseCombineDocumentsChain):
         Combine by mapping first chain over all documents, then reducing the results.
         This reducing can be done recursively if needed (if there are many documents).
         """
-        results = self.llm_chain.apply(
+        map_results = self.llm_chain.apply(
             # FYI - this is parallelized and so it is fast.
             [{self.document_variable_name: d.page_content, **kwargs} for d in docs],
             callbacks=callbacks,
         )
-        return self._process_results(
-            results, docs, token_max, callbacks=callbacks, **kwargs
+        question_result_key = self.llm_chain.output_key
+        result_docs = [
+            Document(page_content=r[question_result_key], metadata=docs[i].metadata)
+            # This uses metadata from the docs, and the textual results from `results`
+            for i, r in enumerate(map_results)
+        ]
+        result, extra_return_dict = self.reduce_documents_chain.combine_docs(
+            result_docs, callbacks=callbacks, **kwargs
         )
+        if self.return_intermediate_steps:
+            intermediate_steps = [r[question_result_key] for r in map_results]
+            extra_return_dict["intermediate_steps"] = intermediate_steps
+        return result, extra_return_dict
 
     async def acombine_docs(
         self, docs: List[Document], callbacks: Callbacks = None, **kwargs: Any
@@ -158,83 +234,24 @@ class MapReduceDocumentsChain(BaseCombineDocumentsChain):
         Combine by mapping first chain over all documents, then reducing the results.
         This reducing can be done recursively if needed (if there are many documents).
         """
-        results = await self.llm_chain.aapply(
+        map_results = await self.llm_chain.aapply(
             # FYI - this is parallelized and so it is fast.
             [{**{self.document_variable_name: d.page_content}, **kwargs} for d in docs],
             callbacks=callbacks,
         )
-        return await self._aprocess_results(
-            results, docs, callbacks=callbacks, **kwargs
-        )
-
-    def _process_results_common(
-        self,
-        results: List[Dict],
-        docs: List[Document],
-        token_max: int = 3000,
-        callbacks: Callbacks = None,
-        **kwargs: Any,
-    ) -> Tuple[List[Document], dict]:
         question_result_key = self.llm_chain.output_key
         result_docs = [
             Document(page_content=r[question_result_key], metadata=docs[i].metadata)
             # This uses metadata from the docs, and the textual results from `results`
-            for i, r in enumerate(results)
+            for i, r in enumerate(map_results)
         ]
-        length_func = self.combine_document_chain.prompt_length
-        num_tokens = length_func(result_docs, **kwargs)
-
-        def _collapse_docs_func(docs: List[Document], **kwargs: Any) -> str:
-            return self._collapse_chain.run(
-                input_documents=docs, callbacks=callbacks, **kwargs
-            )
-
-        while num_tokens is not None and num_tokens > token_max:
-            new_result_doc_list = _split_list_of_docs(
-                result_docs, length_func, token_max, **kwargs
-            )
-            result_docs = []
-            for docs in new_result_doc_list:
-                new_doc = _collapse_docs(docs, _collapse_docs_func, **kwargs)
-                result_docs.append(new_doc)
-            num_tokens = length_func(result_docs, **kwargs)
-        if self.return_intermediate_steps:
-            _results = [r[self.llm_chain.output_key] for r in results]
-            extra_return_dict = {"intermediate_steps": _results}
-        else:
-            extra_return_dict = {}
-        return result_docs, extra_return_dict
-
-    def _process_results(
-        self,
-        results: List[Dict],
-        docs: List[Document],
-        token_max: int = 3000,
-        callbacks: Callbacks = None,
-        **kwargs: Any,
-    ) -> Tuple[str, dict]:
-        result_docs, extra_return_dict = self._process_results_common(
-            results, docs, token_max, callbacks=callbacks, **kwargs
+        result, extra_return_dict = await self.reduce_documents_chain.acombine_docs(
+            result_docs, callbacks=callbacks, **kwargs
         )
-        output = self.combine_document_chain.run(
-            input_documents=result_docs, callbacks=callbacks, **kwargs
-        )
-        return output, extra_return_dict
-
-    async def _aprocess_results(
-        self,
-        results: List[Dict],
-        docs: List[Document],
-        callbacks: Callbacks = None,
-        **kwargs: Any,
-    ) -> Tuple[str, dict]:
-        result_docs, extra_return_dict = self._process_results_common(
-            results, docs, callbacks=callbacks, **kwargs
-        )
-        output = await self.combine_document_chain.arun(
-            input_documents=result_docs, callbacks=callbacks, **kwargs
-        )
-        return output, extra_return_dict
+        if self.return_intermediate_steps:
+            intermediate_steps = [r[question_result_key] for r in map_results]
+            extra_return_dict["intermediate_steps"] = intermediate_steps
+        return result, extra_return_dict
 
     @property
     def _chain_type(self) -> str:
diff --git a/langchain/chains/combine_documents/map_rerank.py b/langchain/chains/combine_documents/map_rerank.py
index ad8409c343..e2d656d07e 100644
--- a/langchain/chains/combine_documents/map_rerank.py
+++ b/langchain/chains/combine_documents/map_rerank.py
@@ -14,7 +14,48 @@ from langchain.output_parsers.regex import RegexParser
 
 
 class MapRerankDocumentsChain(BaseCombineDocumentsChain):
-    """Combining documents by mapping a chain over them, then reranking results."""
+    """Combining documents by mapping a chain over them, then reranking results.
+
+    This algorithm calls an LLMChain on each input document. The LLMChain is expected
+    to have an OutputParser that parses the result into both an answer (`answer_key`)
+    and a score (`rank_key`). The answer with the highest score is then returned.
+
+    Example:
+        .. code-block:: python
+
+            from langchain.chains import StuffDocumentsChain, LLMChain
+            from langchain.prompts import PromptTemplate
+            from langchain.llms import OpenAI
+            from langchain.output_parsers.regex import RegexParser
+
+            document_variable_name = "context"
+            llm = OpenAI()
+            # The prompt here should take as an input variable the
+            # `document_variable_name`
+            # The actual prompt will need to be a lot more complex, this is just
+            # an example.
+            prompt_template = (
+                "Use the following context to tell me the chemical formula "
+                "for water. Output both your answer and a score of how confident "
+                "you are. Context: {content}"
+            )
+            output_parser = RegexParser(
+                regex=r"(.*?)\nScore: (.*)",
+                output_keys=["answer", "score"],
+            )
+            prompt = PromptTemplate(
+                template=prompt_template,
+                input_variables=["context"],
+                output_parser=output_parser,
+            )
+            llm_chain = LLMChain(llm=llm, prompt=prompt)
+            chain = MapRerankDocumentsChain(
+                llm_chain=llm_chain,
+                document_variable_name=document_variable_name,
+                rank_key="score",
+                answer_key="answer",
+            )
+    """
 
     llm_chain: LLMChain
     """Chain to apply to each document individually."""
@@ -26,7 +67,10 @@ class MapRerankDocumentsChain(BaseCombineDocumentsChain):
     answer_key: str
     """Key in output of llm_chain to return as answer."""
     metadata_keys: Optional[List[str]] = None
+    """Additional metadata from the chosen document to return."""
     return_intermediate_steps: bool = False
+    """Return intermediate steps.
+    Intermediate steps include the results of calling llm_chain on each document."""
 
     class Config:
         """Configuration for this pydantic object."""
@@ -96,6 +140,16 @@ class MapRerankDocumentsChain(BaseCombineDocumentsChain):
         """Combine documents in a map rerank manner.
 
         Combine by mapping first chain over all documents, then reranking the results.
+
+        Args:
+            docs: List of documents to combine
+            callbacks: Callbacks to be passed through
+            **kwargs: additional parameters to be passed to LLM calls (like other
+                input variables besides the documents)
+
+        Returns:
+            The first element returned is the single string output. The second
+            element returned is a dictionary of other keys to return.
         """
         results = self.llm_chain.apply_and_parse(
             # FYI - this is parallelized and so it is fast.
@@ -110,6 +164,16 @@ class MapRerankDocumentsChain(BaseCombineDocumentsChain):
         """Combine documents in a map rerank manner.
 
         Combine by mapping first chain over all documents, then reranking the results.
+
+        Args:
+            docs: List of documents to combine
+            callbacks: Callbacks to be passed through
+            **kwargs: additional parameters to be passed to LLM calls (like other
+                input variables besides the documents)
+
+        Returns:
+            The first element returned is the single string output. The second
+            element returned is a dictionary of other keys to return.
         """
         results = await self.llm_chain.aapply_and_parse(
             # FYI - this is parallelized and so it is fast.
diff --git a/langchain/chains/combine_documents/reduce.py b/langchain/chains/combine_documents/reduce.py
new file mode 100644
index 0000000000..e4f1e7719f
--- /dev/null
+++ b/langchain/chains/combine_documents/reduce.py
@@ -0,0 +1,277 @@
+"""Combine many documents together by recursively reducing them."""
+
+from __future__ import annotations
+
+from typing import Any, Callable, List, Optional, Protocol, Tuple
+
+from pydantic import Extra
+
+from langchain.callbacks.manager import Callbacks
+from langchain.chains.combine_documents.base import BaseCombineDocumentsChain
+from langchain.docstore.document import Document
+
+
+class CombineDocsProtocol(Protocol):
+    """Interface for the combine_docs method."""
+
+    def __call__(self, docs: List[Document], **kwargs: Any) -> str:
+        """Interface for the combine_docs method."""
+
+
+class AsyncCombineDocsProtocol(Protocol):
+    """Interface for the combine_docs method."""
+
+    async def __call__(self, docs: List[Document], **kwargs: Any) -> str:
+        """Async nterface for the combine_docs method."""
+
+
+def _split_list_of_docs(
+    docs: List[Document], length_func: Callable, token_max: int, **kwargs: Any
+) -> List[List[Document]]:
+    new_result_doc_list = []
+    _sub_result_docs = []
+    for doc in docs:
+        _sub_result_docs.append(doc)
+        _num_tokens = length_func(_sub_result_docs, **kwargs)
+        if _num_tokens > token_max:
+            if len(_sub_result_docs) == 1:
+                raise ValueError(
+                    "A single document was longer than the context length,"
+                    " we cannot handle this."
+                )
+            new_result_doc_list.append(_sub_result_docs[:-1])
+            _sub_result_docs = _sub_result_docs[-1:]
+    new_result_doc_list.append(_sub_result_docs)
+    return new_result_doc_list
+
+
+def _collapse_docs(
+    docs: List[Document],
+    combine_document_func: CombineDocsProtocol,
+    **kwargs: Any,
+) -> Document:
+    result = combine_document_func(docs, **kwargs)
+    combined_metadata = {k: str(v) for k, v in docs[0].metadata.items()}
+    for doc in docs[1:]:
+        for k, v in doc.metadata.items():
+            if k in combined_metadata:
+                combined_metadata[k] += f", {v}"
+            else:
+                combined_metadata[k] = str(v)
+    return Document(page_content=result, metadata=combined_metadata)
+
+
+async def _acollapse_docs(
+    docs: List[Document],
+    combine_document_func: AsyncCombineDocsProtocol,
+    **kwargs: Any,
+) -> Document:
+    result = await combine_document_func(docs, **kwargs)
+    combined_metadata = {k: str(v) for k, v in docs[0].metadata.items()}
+    for doc in docs[1:]:
+        for k, v in doc.metadata.items():
+            if k in combined_metadata:
+                combined_metadata[k] += f", {v}"
+            else:
+                combined_metadata[k] = str(v)
+    return Document(page_content=result, metadata=combined_metadata)
+
+
+class ReduceDocumentsChain(BaseCombineDocumentsChain):
+    """Combining documents by recursively reducing them.
+
+    This involves
+    - combine_documents_chain
+    - collapse_documents_chain
+
+    `combine_documents_chain` is ALWAYS provided. This is final chain that is called.
+    We pass all previous results to this chain, and the output of this chain is
+    returned as a final result.
+
+    `collapse_documents_chain` is used if the documents passed in are too many to all
+    be passed to `combine_documents_chain` in one go. In this case,
+    `collapse_documents_chain` is called recursively on as big of groups of documents
+    as are allowed.
+
+    Example:
+        .. code-block:: python
+
+            from langchain.chains import (
+                StuffDocumentsChain, LLMChain, ReduceDocumentsChain
+            )
+            from langchain.prompts import PromptTemplate
+            from langchain.llms import OpenAI
+
+            # This controls how each document will be formatted. Specifically,
+            # it will be passed to `format_document` - see that function for more
+            # details.
+            document_prompt = PromptTemplate(
+                input_variables=["page_content"],
+                 template="{page_content}"
+            )
+            document_variable_name = "context"
+            llm = OpenAI()
+            # The prompt here should take as an input variable the
+            # `document_variable_name`
+            prompt = PromptTemplate.from_template(
+                "Summarize this content: {context}"
+            )
+            llm_chain = LLMChain(llm=llm, prompt=prompt)
+            combine_documents_chain = StuffDocumentsChain(
+                llm_chain=llm_chain,
+                document_prompt=document_prompt,
+                document_variable_name=document_variable_name
+            )
+            chain = ReduceDocumentsChain(
+                combine_documents_chain=combine_documents_chain,
+            )
+            # If we wanted to, we could also pass in collapse_documents_chain
+            # which is specifically aimed at collapsing documents BEFORE
+            # the final call.
+            prompt = PromptTemplate.from_template(
+                "Collapse this content: {context}"
+            )
+            llm_chain = LLMChain(llm=llm, prompt=prompt)
+            collapse_documents_chain = StuffDocumentsChain(
+                llm_chain=llm_chain,
+                document_prompt=document_prompt,
+                document_variable_name=document_variable_name
+            )
+            chain = ReduceDocumentsChain(
+                combine_documents_chain=combine_documents_chain,
+                collapse_documents_chain=collapse_documents_chain,
+            )
+    """
+
+    combine_documents_chain: BaseCombineDocumentsChain
+    """Final chain to call to combine documents.
+    This is typically a StuffDocumentsChain."""
+    collapse_documents_chain: Optional[BaseCombineDocumentsChain] = None
+    """Chain to use to collapse documents if needed until they can all fit.
+    If None, will use the combine_documents_chain.
+    This is typically a StuffDocumentsChain."""
+
+    class Config:
+        """Configuration for this pydantic object."""
+
+        extra = Extra.forbid
+        arbitrary_types_allowed = True
+
+    @property
+    def _collapse_chain(self) -> BaseCombineDocumentsChain:
+        if self.collapse_documents_chain is not None:
+            return self.collapse_documents_chain
+        else:
+            return self.combine_documents_chain
+
+    def combine_docs(
+        self,
+        docs: List[Document],
+        token_max: int = 3000,
+        callbacks: Callbacks = None,
+        **kwargs: Any,
+    ) -> Tuple[str, dict]:
+        """Combine multiple documents recursively.
+
+        Args:
+            docs: List of documents to combine, assumed that each one is less than
+                `token_max`.
+            token_max: Recursively creates groups of documents less than this number
+                of tokens.
+            callbacks: Callbacks to be passed through
+            **kwargs: additional parameters to be passed to LLM calls (like other
+                input variables besides the documents)
+
+        Returns:
+            The first element returned is the single string output. The second
+            element returned is a dictionary of other keys to return.
+        """
+        result_docs, extra_return_dict = self._collapse(
+            docs, token_max, callbacks=callbacks, **kwargs
+        )
+        return self.combine_documents_chain.combine_docs(
+            docs=result_docs, callbacks=callbacks, **kwargs
+        )
+
+    async def acombine_docs(
+        self, docs: List[Document], callbacks: Callbacks = None, **kwargs: Any
+    ) -> Tuple[str, dict]:
+        """Combine multiple documents recursively.
+
+        Args:
+            docs: List of documents to combine, assumed that each one is less than
+                `token_max`.
+            token_max: Recursively creates groups of documents less than this number
+                of tokens.
+            callbacks: Callbacks to be passed through
+            **kwargs: additional parameters to be passed to LLM calls (like other
+                input variables besides the documents)
+
+        Returns:
+            The first element returned is the single string output. The second
+            element returned is a dictionary of other keys to return.
+        """
+        result_docs, extra_return_dict = await self._acollapse(
+            docs, callbacks=callbacks, **kwargs
+        )
+        return await self.combine_documents_chain.acombine_docs(
+            docs=result_docs, callbacks=callbacks, **kwargs
+        )
+
+    def _collapse(
+        self,
+        docs: List[Document],
+        token_max: int = 3000,
+        callbacks: Callbacks = None,
+        **kwargs: Any,
+    ) -> Tuple[List[Document], dict]:
+        result_docs = docs
+        length_func = self.combine_documents_chain.prompt_length
+        num_tokens = length_func(result_docs, **kwargs)
+
+        def _collapse_docs_func(docs: List[Document], **kwargs: Any) -> str:
+            return self._collapse_chain.run(
+                input_documents=docs, callbacks=callbacks, **kwargs
+            )
+
+        while num_tokens is not None and num_tokens > token_max:
+            new_result_doc_list = _split_list_of_docs(
+                result_docs, length_func, token_max, **kwargs
+            )
+            result_docs = []
+            for docs in new_result_doc_list:
+                new_doc = _collapse_docs(docs, _collapse_docs_func, **kwargs)
+                result_docs.append(new_doc)
+            num_tokens = length_func(result_docs, **kwargs)
+        return result_docs, {}
+
+    async def _acollapse(
+        self,
+        docs: List[Document],
+        token_max: int = 3000,
+        callbacks: Callbacks = None,
+        **kwargs: Any,
+    ) -> Tuple[List[Document], dict]:
+        result_docs = docs
+        length_func = self.combine_documents_chain.prompt_length
+        num_tokens = length_func(result_docs, **kwargs)
+
+        async def _collapse_docs_func(docs: List[Document], **kwargs: Any) -> str:
+            return await self._collapse_chain.arun(
+                input_documents=docs, callbacks=callbacks, **kwargs
+            )
+
+        while num_tokens is not None and num_tokens > token_max:
+            new_result_doc_list = _split_list_of_docs(
+                result_docs, length_func, token_max, **kwargs
+            )
+            result_docs = []
+            for docs in new_result_doc_list:
+                new_doc = await _acollapse_docs(docs, _collapse_docs_func, **kwargs)
+                result_docs.append(new_doc)
+            num_tokens = length_func(result_docs, **kwargs)
+        return result_docs, {}
+
+    @property
+    def _chain_type(self) -> str:
+        return "reduce_documents_chain"
diff --git a/langchain/chains/combine_documents/refine.py b/langchain/chains/combine_documents/refine.py
index fac234719c..18fb0ed097 100644
--- a/langchain/chains/combine_documents/refine.py
+++ b/langchain/chains/combine_documents/refine.py
@@ -9,12 +9,11 @@ from pydantic import Extra, Field, root_validator
 from langchain.callbacks.manager import Callbacks
 from langchain.chains.combine_documents.base import (
     BaseCombineDocumentsChain,
-    format_document,
 )
 from langchain.chains.llm import LLMChain
 from langchain.docstore.document import Document
 from langchain.prompts.prompt import PromptTemplate
-from langchain.schema import BasePromptTemplate
+from langchain.schema import BasePromptTemplate, format_document
 
 
 def _get_default_document_prompt() -> PromptTemplate:
@@ -22,7 +21,55 @@ def _get_default_document_prompt() -> PromptTemplate:
 
 
 class RefineDocumentsChain(BaseCombineDocumentsChain):
-    """Combine documents by doing a first pass and then refining on more documents."""
+    """Combine documents by doing a first pass and then refining on more documents.
+
+    This algorithm first calls `initial_llm_chain` on the first document, passing
+    that first document in with the variable name `document_variable_name`, and
+    produces a new variable with the variable name `initial_response_name`.
+
+    Then, it loops over every remaining document. This is called the "refine" step.
+    It calls `refine_llm_chain`,
+    passing in that document with the variable name `document_variable_name`
+    as well as the previous response with the variable name `initial_response_name`.
+
+    Example:
+        .. code-block:: python
+
+            from langchain.chains import RefineDocumentsChain, LLMChain
+            from langchain.prompts import PromptTemplate
+            from langchain.llms import OpenAI
+
+            # This controls how each document will be formatted. Specifically,
+            # it will be passed to `format_document` - see that function for more
+            # details.
+            document_prompt = PromptTemplate(
+                input_variables=["page_content"],
+                 template="{page_content}"
+            )
+            document_variable_name = "context"
+            llm = OpenAI()
+            # The prompt here should take as an input variable the
+            # `document_variable_name`
+            prompt = PromptTemplate.from_template(
+                "Summarize this content: {context}"
+            )
+            llm_chain = LLMChain(llm=llm, prompt=prompt)
+            initial_response_name = "prev_response"
+            # The prompt here should take as an input variable the
+            # `document_variable_name` as well as `initial_response_name`
+            prompt_refine = PromptTemplate.from_template(
+                "Here's your first summary: {prev_response}. "
+                "Now add to it based on the following context: {context}"
+            )
+            llm_chain_refine = LLMChain(llm=llm, prompt=prompt_refine)
+            chain = RefineDocumentsChain(
+                initial_llm_chain=initial_llm_chain,
+                refine_llm_chain=refine_llm_chain,
+                document_prompt=document_prompt,
+                document_variable_name=document_variable_name,
+                initial_response_name=initial_response_name,
+            )
+    """
 
     initial_llm_chain: LLMChain
     """LLM chain to use on initial document."""
@@ -36,7 +83,7 @@ class RefineDocumentsChain(BaseCombineDocumentsChain):
     document_prompt: BasePromptTemplate = Field(
         default_factory=_get_default_document_prompt
     )
-    """Prompt to use to format each document."""
+    """Prompt to use to format each document, gets passed to `format_document`."""
     return_intermediate_steps: bool = False
     """Return the results of the refine steps in the output."""
 
@@ -89,7 +136,18 @@ class RefineDocumentsChain(BaseCombineDocumentsChain):
     def combine_docs(
         self, docs: List[Document], callbacks: Callbacks = None, **kwargs: Any
     ) -> Tuple[str, dict]:
-        """Combine by mapping first chain over all, then stuffing into final chain."""
+        """Combine by mapping first chain over all, then stuffing into final chain.
+
+        Args:
+            docs: List of documents to combine
+            callbacks: Callbacks to be passed through
+            **kwargs: additional parameters to be passed to LLM calls (like other
+                input variables besides the documents)
+
+        Returns:
+            The first element returned is the single string output. The second
+            element returned is a dictionary of other keys to return.
+        """
         inputs = self._construct_initial_inputs(docs, **kwargs)
         res = self.initial_llm_chain.predict(callbacks=callbacks, **inputs)
         refine_steps = [res]
@@ -103,7 +161,18 @@ class RefineDocumentsChain(BaseCombineDocumentsChain):
     async def acombine_docs(
         self, docs: List[Document], callbacks: Callbacks = None, **kwargs: Any
     ) -> Tuple[str, dict]:
-        """Combine by mapping first chain over all, then stuffing into final chain."""
+        """Combine by mapping first chain over all, then stuffing into final chain.
+
+        Args:
+            docs: List of documents to combine
+            callbacks: Callbacks to be passed through
+            **kwargs: additional parameters to be passed to LLM calls (like other
+                input variables besides the documents)
+
+        Returns:
+            The first element returned is the single string output. The second
+            element returned is a dictionary of other keys to return.
+        """
         inputs = self._construct_initial_inputs(docs, **kwargs)
         res = await self.initial_llm_chain.apredict(callbacks=callbacks, **inputs)
         refine_steps = [res]
diff --git a/langchain/chains/combine_documents/stuff.py b/langchain/chains/combine_documents/stuff.py
index e4859ff6ca..6712734231 100644
--- a/langchain/chains/combine_documents/stuff.py
+++ b/langchain/chains/combine_documents/stuff.py
@@ -7,12 +7,11 @@ from pydantic import Extra, Field, root_validator
 from langchain.callbacks.manager import Callbacks
 from langchain.chains.combine_documents.base import (
     BaseCombineDocumentsChain,
-    format_document,
 )
 from langchain.chains.llm import LLMChain
 from langchain.docstore.document import Document
 from langchain.prompts.prompt import PromptTemplate
-from langchain.schema import BasePromptTemplate
+from langchain.schema import BasePromptTemplate, format_document
 
 
 def _get_default_document_prompt() -> PromptTemplate:
@@ -20,14 +19,50 @@ def _get_default_document_prompt() -> PromptTemplate:
 
 
 class StuffDocumentsChain(BaseCombineDocumentsChain):
-    """Chain that combines documents by stuffing into context."""
+    """Chain that combines documents by stuffing into context.
+
+    This chain takes a list of documents and first combines them into a single string.
+    It does this by formatting each document into a string with the `document_prompt`
+    and then joining them together with `document_separator`. It then adds that new
+    string to the inputs with the variable name set by `document_variable_name`.
+    Those inputs are then passed to the `llm_chain`.
+
+    Example:
+        .. code-block:: python
+
+            from langchain.chains import StuffDocumentsChain, LLMChain
+            from langchain.prompts import PromptTemplate
+            from langchain.llms import OpenAI
+
+            # This controls how each document will be formatted. Specifically,
+            # it will be passed to `format_document` - see that function for more
+            # details.
+            document_prompt = PromptTemplate(
+                input_variables=["page_content"],
+                 template="{page_content}"
+            )
+            document_variable_name = "context"
+            llm = OpenAI()
+            # The prompt here should take as an input variable the
+            # `document_variable_name`
+            prompt = PromptTemplate.from_template(
+                "Summarize this content: {context}"
+            )
+            llm_chain = LLMChain(llm=llm, prompt=prompt)
+            chain = StuffDocumentsChain(
+                llm_chain=llm_chain,
+                document_prompt=document_prompt,
+                document_variable_name=document_variable_name
+            )
+    """
 
     llm_chain: LLMChain
-    """LLM wrapper to use after formatting documents."""
+    """LLM chain which is called with the formatted document string,
+    along with any other inputs."""
     document_prompt: BasePromptTemplate = Field(
         default_factory=_get_default_document_prompt
     )
-    """Prompt to use to format each document."""
+    """Prompt to use to format each document, gets passed to `format_document`."""
     document_variable_name: str
     """The variable name in the llm_chain to put the documents in.
     If only one variable in the llm_chain, this need not be provided."""
@@ -42,7 +77,12 @@ class StuffDocumentsChain(BaseCombineDocumentsChain):
 
     @root_validator(pre=True)
     def get_default_document_variable_name(cls, values: Dict) -> Dict:
-        """Get default document variable name, if not provided."""
+        """Get default document variable name, if not provided.
+
+        If only one variable is present in the llm_chain.prompt,
+        we can infer that the formatted documents should be passed in
+        with this variable name.
+        """
         llm_chain_variables = values["llm_chain"].prompt.input_variables
         if "document_variable_name" not in values:
             if len(llm_chain_variables) == 1:
@@ -61,6 +101,20 @@ class StuffDocumentsChain(BaseCombineDocumentsChain):
         return values
 
     def _get_inputs(self, docs: List[Document], **kwargs: Any) -> dict:
+        """Construct inputs from kwargs and docs.
+
+        Format and the join all the documents together into one input with name
+        `self.document_variable_name`. The pluck any additional variables
+        from **kwargs.
+
+        Args:
+            docs: List of documents to format and then join into single input
+            **kwargs: additional inputs to chain, will pluck any other required
+                arguments from here.
+
+        Returns:
+            dictionary of inputs to LLMChain
+        """
         # Format each document according to the prompt
         doc_strings = [format_document(doc, self.document_prompt) for doc in docs]
         # Join the documents together to put them in the prompt.
@@ -73,7 +127,21 @@ class StuffDocumentsChain(BaseCombineDocumentsChain):
         return inputs
 
     def prompt_length(self, docs: List[Document], **kwargs: Any) -> Optional[int]:
-        """Get the prompt length by formatting the prompt."""
+        """Return the prompt length given the documents passed in.
+
+        This can be used by a caller to determine whether passing in a list
+        of documents would exceed a certain prompt length. This useful when
+        trying to ensure that the size of a prompt remains below a certain
+        context limit.
+
+        Args:
+            docs: List[Document], a list of documents to use to calculate the
+                total prompt length.
+
+        Returns:
+            Returns None if the method does not depend on the prompt length,
+            otherwise the length of the prompt in tokens.
+        """
         inputs = self._get_inputs(docs, **kwargs)
         prompt = self.llm_chain.prompt.format(**inputs)
         return self.llm_chain.llm.get_num_tokens(prompt)
@@ -81,7 +149,17 @@ class StuffDocumentsChain(BaseCombineDocumentsChain):
     def combine_docs(
         self, docs: List[Document], callbacks: Callbacks = None, **kwargs: Any
     ) -> Tuple[str, dict]:
-        """Stuff all documents into one prompt and pass to LLM."""
+        """Stuff all documents into one prompt and pass to LLM.
+
+        Args:
+            docs: List of documents to join together into one variable
+            callbacks: Optional callbacks to pass along
+            **kwargs: additional parameters to use to get inputs to LLMChain.
+
+        Returns:
+            The first element returned is the single string output. The second
+            element returned is a dictionary of other keys to return.
+        """
         inputs = self._get_inputs(docs, **kwargs)
         # Call predict on the LLM.
         return self.llm_chain.predict(callbacks=callbacks, **inputs), {}
@@ -89,7 +167,17 @@ class StuffDocumentsChain(BaseCombineDocumentsChain):
     async def acombine_docs(
         self, docs: List[Document], callbacks: Callbacks = None, **kwargs: Any
     ) -> Tuple[str, dict]:
-        """Stuff all documents into one prompt and pass to LLM."""
+        """Stuff all documents into one prompt and pass to LLM.
+
+        Args:
+            docs: List of documents to join together into one variable
+            callbacks: Optional callbacks to pass along
+            **kwargs: additional parameters to use to get inputs to LLMChain.
+
+        Returns:
+            The first element returned is the single string output. The second
+            element returned is a dictionary of other keys to return.
+        """
         inputs = self._get_inputs(docs, **kwargs)
         # Call predict on the LLM.
         return await self.llm_chain.apredict(callbacks=callbacks, **inputs), {}
diff --git a/langchain/chains/loading.py b/langchain/chains/loading.py
index a01872fbe8..75b2eaaf8b 100644
--- a/langchain/chains/loading.py
+++ b/langchain/chains/loading.py
@@ -5,6 +5,7 @@ from typing import Any, Union
 
 import yaml
 
+from langchain.chains import ReduceDocumentsChain
 from langchain.chains.api.base import APIChain
 from langchain.chains.base import Chain
 from langchain.chains.combine_documents.map_reduce import MapReduceDocumentsChain
@@ -117,9 +118,9 @@ def _load_map_reduce_documents_chain(
 
     if "combine_document_chain" in config:
         combine_document_chain_config = config.pop("combine_document_chain")
-        combine_document_chain = load_chain_from_config(combine_document_chain_config)
+        combine_documents_chain = load_chain_from_config(combine_document_chain_config)
     elif "combine_document_chain_path" in config:
-        combine_document_chain = load_chain(config.pop("combine_document_chain_path"))
+        combine_documents_chain = load_chain(config.pop("combine_document_chain_path"))
     else:
         raise ValueError(
             "One of `combine_document_chain` or "
@@ -128,17 +129,24 @@ def _load_map_reduce_documents_chain(
     if "collapse_document_chain" in config:
         collapse_document_chain_config = config.pop("collapse_document_chain")
         if collapse_document_chain_config is None:
-            collapse_document_chain = None
+            collapse_documents_chain = None
         else:
-            collapse_document_chain = load_chain_from_config(
+            collapse_documents_chain = load_chain_from_config(
                 collapse_document_chain_config
             )
     elif "collapse_document_chain_path" in config:
-        collapse_document_chain = load_chain(config.pop("collapse_document_chain_path"))
+        collapse_documents_chain = load_chain(
+            config.pop("collapse_document_chain_path")
+        )
+    else:
+        collapse_documents_chain = None
+    reduce_documents_chain = ReduceDocumentsChain(
+        combine_documents_chain=combine_documents_chain,
+        collapse_documents_chain=collapse_documents_chain,
+    )
     return MapReduceDocumentsChain(
         llm_chain=llm_chain,
-        combine_document_chain=combine_document_chain,
-        collapse_document_chain=collapse_document_chain,
+        reduce_documents_chain=reduce_documents_chain,
         **config,
     )
 
diff --git a/langchain/chains/mapreduce.py b/langchain/chains/mapreduce.py
index 732d489e87..8b217bf023 100644
--- a/langchain/chains/mapreduce.py
+++ b/langchain/chains/mapreduce.py
@@ -11,6 +11,7 @@ from pydantic import Extra
 
 from langchain.base_language import BaseLanguageModel
 from langchain.callbacks.manager import CallbackManagerForChainRun, Callbacks
+from langchain.chains import ReduceDocumentsChain
 from langchain.chains.base import Chain
 from langchain.chains.combine_documents.base import BaseCombineDocumentsChain
 from langchain.chains.combine_documents.map_reduce import MapReduceDocumentsChain
@@ -44,14 +45,17 @@ class MapReduceChain(Chain):
     ) -> MapReduceChain:
         """Construct a map-reduce chain that uses the chain for map and reduce."""
         llm_chain = LLMChain(llm=llm, prompt=prompt, callbacks=callbacks)
-        reduce_chain = StuffDocumentsChain(
+        stuff_chain = StuffDocumentsChain(
             llm_chain=llm_chain,
             callbacks=callbacks,
             **(reduce_chain_kwargs if reduce_chain_kwargs else {}),
         )
+        reduce_documents_chain = ReduceDocumentsChain(
+            combine_documents_chain=stuff_chain
+        )
         combine_documents_chain = MapReduceDocumentsChain(
             llm_chain=llm_chain,
-            combine_document_chain=reduce_chain,
+            reduce_documents_chain=reduce_documents_chain,
             callbacks=callbacks,
             **(combine_chain_kwargs if combine_chain_kwargs else {}),
         )
diff --git a/langchain/chains/qa_with_sources/base.py b/langchain/chains/qa_with_sources/base.py
index ba882310b6..5e1799cc29 100644
--- a/langchain/chains/qa_with_sources/base.py
+++ b/langchain/chains/qa_with_sources/base.py
@@ -14,6 +14,7 @@ from langchain.callbacks.manager import (
     AsyncCallbackManagerForChainRun,
     CallbackManagerForChainRun,
 )
+from langchain.chains import ReduceDocumentsChain
 from langchain.chains.base import Chain
 from langchain.chains.combine_documents.base import BaseCombineDocumentsChain
 from langchain.chains.combine_documents.map_reduce import MapReduceDocumentsChain
@@ -58,13 +59,16 @@ class BaseQAWithSourcesChain(Chain, ABC):
             document_prompt=document_prompt,
             document_variable_name="summaries",
         )
-        combine_document_chain = MapReduceDocumentsChain(
+        reduce_documents_chain = ReduceDocumentsChain(
+            combine_documents_chain=combine_results_chain
+        )
+        combine_documents_chain = MapReduceDocumentsChain(
             llm_chain=llm_question_chain,
-            combine_document_chain=combine_results_chain,
+            reduce_documents_chain=reduce_documents_chain,
             document_variable_name="context",
         )
         return cls(
-            combine_documents_chain=combine_document_chain,
+            combine_documents_chain=combine_documents_chain,
             **kwargs,
         )
 
@@ -78,10 +82,10 @@ class BaseQAWithSourcesChain(Chain, ABC):
     ) -> BaseQAWithSourcesChain:
         """Load chain from chain type."""
         _chain_kwargs = chain_type_kwargs or {}
-        combine_document_chain = load_qa_with_sources_chain(
+        combine_documents_chain = load_qa_with_sources_chain(
             llm, chain_type=chain_type, **_chain_kwargs
         )
-        return cls(combine_documents_chain=combine_document_chain, **kwargs)
+        return cls(combine_documents_chain=combine_documents_chain, **kwargs)
 
     class Config:
         """Configuration for this pydantic object."""
@@ -110,7 +114,7 @@ class BaseQAWithSourcesChain(Chain, ABC):
 
     @root_validator(pre=True)
     def validate_naming(cls, values: Dict) -> Dict:
-        """Fix backwards compatability in naming."""
+        """Fix backwards compatibility in naming."""
         if "combine_document_chain" in values:
             values["combine_documents_chain"] = values.pop("combine_document_chain")
         return values
diff --git a/langchain/chains/qa_with_sources/loading.py b/langchain/chains/qa_with_sources/loading.py
index 57ea76a763..9a5058938c 100644
--- a/langchain/chains/qa_with_sources/loading.py
+++ b/langchain/chains/qa_with_sources/loading.py
@@ -4,6 +4,7 @@ from __future__ import annotations
 from typing import Any, Mapping, Optional, Protocol
 
 from langchain.base_language import BaseLanguageModel
+from langchain.chains import ReduceDocumentsChain
 from langchain.chains.combine_documents.base import BaseCombineDocumentsChain
 from langchain.chains.combine_documents.map_reduce import MapReduceDocumentsChain
 from langchain.chains.combine_documents.map_rerank import MapRerankDocumentsChain
@@ -83,7 +84,7 @@ def _load_map_reduce_chain(
     map_chain = LLMChain(llm=llm, prompt=question_prompt, verbose=verbose)
     _reduce_llm = reduce_llm or llm
     reduce_chain = LLMChain(llm=_reduce_llm, prompt=combine_prompt, verbose=verbose)
-    combine_document_chain = StuffDocumentsChain(
+    combine_documents_chain = StuffDocumentsChain(
         llm_chain=reduce_chain,
         document_variable_name=combine_document_variable_name,
         document_prompt=document_prompt,
@@ -107,11 +108,14 @@ def _load_map_reduce_chain(
             document_variable_name=combine_document_variable_name,
             document_prompt=document_prompt,
         )
+    reduce_documents_chain = ReduceDocumentsChain(
+        combine_documents_chain=combine_documents_chain,
+        collapse_documents_chain=collapse_chain,
+    )
     return MapReduceDocumentsChain(
         llm_chain=map_chain,
-        combine_document_chain=combine_document_chain,
+        reduce_documents_chain=reduce_documents_chain,
         document_variable_name=map_reduce_document_variable_name,
-        collapse_document_chain=collapse_chain,
         verbose=verbose,
         **kwargs,
     )
diff --git a/langchain/chains/question_answering/__init__.py b/langchain/chains/question_answering/__init__.py
index a6eb09e232..9fa2d2bffd 100644
--- a/langchain/chains/question_answering/__init__.py
+++ b/langchain/chains/question_answering/__init__.py
@@ -4,6 +4,7 @@ from typing import Any, Mapping, Optional, Protocol
 from langchain.base_language import BaseLanguageModel
 from langchain.callbacks.base import BaseCallbackManager
 from langchain.callbacks.manager import Callbacks
+from langchain.chains import ReduceDocumentsChain
 from langchain.chains.combine_documents.base import BaseCombineDocumentsChain
 from langchain.chains.combine_documents.map_reduce import MapReduceDocumentsChain
 from langchain.chains.combine_documents.map_rerank import MapRerankDocumentsChain
@@ -122,7 +123,7 @@ def _load_map_reduce_chain(
         callbacks=callbacks,
     )
     # TODO: document prompt
-    combine_document_chain = StuffDocumentsChain(
+    combine_documents_chain = StuffDocumentsChain(
         llm_chain=reduce_chain,
         document_variable_name=combine_document_variable_name,
         verbose=verbose,
@@ -150,11 +151,14 @@ def _load_map_reduce_chain(
             verbose=verbose,
             callback_manager=callback_manager,
         )
+    reduce_documents_chain = ReduceDocumentsChain(
+        combine_documents_chain=combine_documents_chain,
+        collapse_documents_chain=collapse_chain,
+    )
     return MapReduceDocumentsChain(
         llm_chain=map_chain,
-        combine_document_chain=combine_document_chain,
         document_variable_name=map_reduce_document_variable_name,
-        collapse_document_chain=collapse_chain,
+        reduce_documents_chain=reduce_documents_chain,
         verbose=verbose,
         callback_manager=callback_manager,
         callbacks=callbacks,
diff --git a/langchain/chains/summarize/__init__.py b/langchain/chains/summarize/__init__.py
index fa88cea1ff..211bf701d9 100644
--- a/langchain/chains/summarize/__init__.py
+++ b/langchain/chains/summarize/__init__.py
@@ -2,6 +2,7 @@
 from typing import Any, Mapping, Optional, Protocol
 
 from langchain.base_language import BaseLanguageModel
+from langchain.chains import ReduceDocumentsChain
 from langchain.chains.combine_documents.base import BaseCombineDocumentsChain
 from langchain.chains.combine_documents.map_reduce import MapReduceDocumentsChain
 from langchain.chains.combine_documents.refine import RefineDocumentsChain
@@ -53,7 +54,7 @@ def _load_map_reduce_chain(
     _reduce_llm = reduce_llm or llm
     reduce_chain = LLMChain(llm=_reduce_llm, prompt=combine_prompt, verbose=verbose)
     # TODO: document prompt
-    combine_document_chain = StuffDocumentsChain(
+    combine_documents_chain = StuffDocumentsChain(
         llm_chain=reduce_chain,
         document_variable_name=combine_document_variable_name,
         verbose=verbose,
@@ -75,11 +76,14 @@ def _load_map_reduce_chain(
             ),
             document_variable_name=combine_document_variable_name,
         )
+    reduce_documents_chain = ReduceDocumentsChain(
+        combine_documents_chain=combine_documents_chain,
+        collapse_documents_chain=collapse_chain,
+    )
     return MapReduceDocumentsChain(
         llm_chain=map_chain,
-        combine_document_chain=combine_document_chain,
+        reduce_documents_chain=reduce_documents_chain,
         document_variable_name=map_reduce_document_variable_name,
-        collapse_document_chain=collapse_chain,
         verbose=verbose,
         **kwargs,
     )
diff --git a/langchain/schema/__init__.py b/langchain/schema/__init__.py
index 13983206bf..818ff4113d 100644
--- a/langchain/schema/__init__.py
+++ b/langchain/schema/__init__.py
@@ -28,7 +28,7 @@ from langchain.schema.output_parser import (
     OutputParserException,
 )
 from langchain.schema.prompt import PromptValue
-from langchain.schema.prompt_template import BasePromptTemplate
+from langchain.schema.prompt_template import BasePromptTemplate, format_document
 from langchain.schema.retriever import BaseRetriever
 
 RUN_KEY = "__run"
@@ -66,4 +66,5 @@ __all__ = [
     "BaseOutputParser",
     "BaseLLMOutputParser",
     "BasePromptTemplate",
+    "format_document",
 ]
diff --git a/langchain/schema/prompt_template.py b/langchain/schema/prompt_template.py
index 6ed048df4a..7b340887eb 100644
--- a/langchain/schema/prompt_template.py
+++ b/langchain/schema/prompt_template.py
@@ -9,7 +9,9 @@ import yaml
 from pydantic import Field, root_validator
 
 from langchain.load.serializable import Serializable
-from langchain.schema import BaseOutputParser, PromptValue
+from langchain.schema.document import Document
+from langchain.schema.output_parser import BaseOutputParser
+from langchain.schema.prompt import PromptValue
 
 
 class BasePromptTemplate(Serializable, ABC):
@@ -137,3 +139,48 @@ class BasePromptTemplate(Serializable, ABC):
                 yaml.dump(prompt_dict, f, default_flow_style=False)
         else:
             raise ValueError(f"{save_path} must be json or yaml")
+
+
+def format_document(doc: Document, prompt: BasePromptTemplate) -> str:
+    """Format a document into a string based on a prompt template.
+
+    First, this pulls information from the document from two sources:
+
+    1. `page_content`: this takes the information from the `document.page_content`
+        and assigns it to a variable named `page_content`.
+    2. metadata: this takes information from `document.metadata` and assigns
+        it to variables of the same name.
+
+    Those variables are then passed into the `prompt` to produce a formatted string.
+
+    Args:
+        doc: Document, the page_content and metadata will be used to create
+            the final string.
+        prompt: BasePromptTemplate, will be used to format the page_content
+            and metadata into the final string.
+
+    Returns:
+        string of the document formatted.
+
+    Example:
+        .. code-block:: python
+        from langchain.schema import Document
+        from langchain.prompts import PromptTemplate
+        doc = Document(page_content="This is a joke", metadata={"page": "1"})
+        prompt = PromptTemplate.from_template("Page {page}: {page_content}")
+        format_document(doc, prompt)
+        >>> "Page 1: This is a joke"
+    """
+    base_info = {"page_content": doc.page_content, **doc.metadata}
+    missing_metadata = set(prompt.input_variables).difference(base_info)
+    if len(missing_metadata) > 0:
+        required_metadata = [
+            iv for iv in prompt.input_variables if iv != "page_content"
+        ]
+        raise ValueError(
+            f"Document prompt requires documents to have metadata variables: "
+            f"{required_metadata}. Received document with missing metadata: "
+            f"{list(missing_metadata)}."
+        )
+    document_info = {k: base_info[k] for k in prompt.input_variables}
+    return prompt.format(**document_info)
diff --git a/tests/unit_tests/chains/test_combine_documents.py b/tests/unit_tests/chains/test_combine_documents.py
index 7377250352..df2212588e 100644
--- a/tests/unit_tests/chains/test_combine_documents.py
+++ b/tests/unit_tests/chains/test_combine_documents.py
@@ -5,12 +5,12 @@ from typing import Any, List
 import pytest
 
 from langchain import PromptTemplate
-from langchain.chains.combine_documents.base import format_document
-from langchain.chains.combine_documents.map_reduce import (
+from langchain.chains.combine_documents.reduce import (
     _collapse_docs,
     _split_list_of_docs,
 )
 from langchain.docstore.document import Document
+from langchain.schema import format_document
 
 
 def _fake_docs_len_func(docs: List[Document]) -> int:
@@ -28,13 +28,6 @@ def test__split_list_long_single_doc() -> None:
         _split_list_of_docs(docs, _fake_docs_len_func, 100)
 
 
-def test__split_list_long_pair_doc() -> None:
-    """Test splitting of a list with two medium docs."""
-    docs = [Document(page_content="foo" * 30)] * 2
-    with pytest.raises(ValueError):
-        _split_list_of_docs(docs, _fake_docs_len_func, 100)
-
-
 def test__split_list_single_doc() -> None:
     """Test splitting works with just a single doc."""
     docs = [Document(page_content="foo")]
diff --git a/tests/unit_tests/test_dependencies.py b/tests/unit_tests/test_dependencies.py
index 72ca1b793f..b200bf30b4 100644
--- a/tests/unit_tests/test_dependencies.py
+++ b/tests/unit_tests/test_dependencies.py
@@ -86,8 +86,8 @@ def test_imports() -> None:
     from langchain.document_loaders import BSHTMLLoader  # noqa: F401
     from langchain.embeddings import OpenAIEmbeddings  # noqa: F401
     from langchain.llms import OpenAI  # noqa: F401
-    from langchain.prompts import BasePromptTemplate  # noqa: F401
     from langchain.retrievers import VespaRetriever  # noqa: F401
+    from langchain.schema import BasePromptTemplate  # noqa: F401
     from langchain.tools import DuckDuckGoSearchResults  # noqa: F401
     from langchain.utilities import SerpAPIWrapper  # noqa: F401
     from langchain.vectorstores import FAISS  # noqa: F401