diff --git a/Documentation/git-patch-id.txt b/Documentation/git-patch-id.txt
index 3e560a37e8..c8bd197779 100644
--- a/Documentation/git-patch-id.txt
+++ b/Documentation/git-patch-id.txt
@@ -18,6 +18,12 @@ ID" are almost guaranteed to be the same thing.
 
 IOW, you can use this thing to look for likely duplicate commits.
 
+When dealing with git-diff-tree output, it takes advantage of
+the fact that the patch is prefixed with the object name of the
+commit, and outputs two 40-byte hexadecimal string.  The first
+string is the patch ID, and the second string is the commit ID.
+This can be used to make a mapping from patch ID to commit ID.
+
 OPTIONS
 -------
 <patch>::