diff --git a/mercurial/help.py b/mercurial/help.py
--- a/mercurial/help.py
+++ b/mercurial/help.py
@@ -320,6 +320,8 @@
      loaddoc('config', subdir='internals')),
     (['extensions', 'extension'], _('Extension API'),
      loaddoc('extensions', subdir='internals')),
+    (['mergestate'], _('Mergestate'),
+     loaddoc('mergestate', subdir='internals')),
     (['requirements'], _('Repository Requirements'),
      loaddoc('requirements', subdir='internals')),
     (['revlogs'], _('Revision Logs'),
diff --git a/mercurial/help/internals/mergestate.txt b/mercurial/help/internals/mergestate.txt
new file mode 100644
--- /dev/null
+++ b/mercurial/help/internals/mergestate.txt
@@ -0,0 +1,80 @@
+The active mergestate is stored in ``.hg/merge`` when a merge is triggered
+by commands like ``hg merge``, ``hg rebase``, etc. until the merge is
+completed or aborted to track the 3-way merge state of individual files.
+
+The contents of the directory are:
+
+Conflicting files
+-----------------
+
+The local version of the conflicting files are stored with their
+filenames as the hash of their paths.
+
+state
+-----
+
+This mergestate file record is used by hg version prior to 2.9.1
+and contains less data than ``state2``. If there is no contradiction
+with ``state2``, we can assume that both are written at the same time.
+In this case, data from ``state2`` is used. Otherwise, we use ``state``.
+We read/write both ``state`` and ``state2`` records to ensure backward
+compatibility.
+
+state2
+------
+
+This record stores a superset of data in ``state``, including new kinds
+of records in the future.
+
+Each record can contain arbitrary content and has an associated type. This
+`type` should be a letter. If `type` is uppercase, the record is mandatory:
+versions of Mercurial that don't support it should abort. If `type` is
+lowercase, the record can be safely ignored.
+
+Currently known records:
+
+| * L: the node of the "local" part of the merge (hexified version)
+| * O: the node of the "other" part of the merge (hexified version)
+| * F: a file to be merged entry
+| * C: a change/delete or delete/change conflict
+| * D: a file that the external merge driver will merge internally
+|      (experimental)
+| * P: a path conflict (file vs directory)
+| * m: the external merge driver defined for this merge plus its run state
+|      (experimental)
+| * f: a (filename, dictionary) tuple of optional values for a given file
+| * X: unsupported mandatory record type (used in tests)
+| * x: unsupported advisory record type (used in tests)
+| * l: the labels for the parts of the merge.
+
+Merge driver run states (experimental):
+
+| * u: driver-resolved files unmarked -- needs to be run next time we're
+|      about to resolve or commit
+| * m: driver-resolved files marked -- only needs to be run before commit
+| * s: success/skipped -- does not need to be run any more
+
+Merge record states (indexed by filename):
+
+| * u: unresolved conflict
+| * r: resolved conflict
+| * pu: unresolved path conflict (file conflicts with directory)
+| * pr: resolved path conflict
+| * d: driver-resolved conflict
+
+The resolve command transitions between 'u' and 'r' for conflicts and
+'pu' and 'pr' for path conflicts.
+
+This format is a list of arbitrary records of the form:
+
+[type][length][content]
+
+`type` is a single character, `length` is a 4 byte integer, and
+`content` is an arbitrary byte sequence of length `length`.
+
+Mercurial versions prior to 3.7 have a bug where if there are
+unsupported mandatory merge records, attempting to clear out the merge
+state with hg update --clean or similar aborts. The 't' record type
+works around that by writing out what those versions treat as an
+advisory record, but later versions interpret as special: the first
+character is the 'real' record type and everything onwards is the data.