--- a/hg.rst	Mon Oct 10 01:07:33 2011 +0300
+++ b/hg.rst	Tue Oct 11 23:05:41 2011 +0300
@@ -408,6 +408,13 @@
   http://mercurial.selenic.com/wiki/TipsAndTricks#Merge_or_rebase_with_uncommitted_changes
   http://mercurial.selenic.com/wiki/ShelveExtension
 
+Find greatest common ancestor of the two changesets.
+====================================================
+
+  $ hg help revsets
+  $ hg log -r "ancestor($rev1,$rev2)"
+  $ hg log -r "ancestor($rev1,ancestor($rev2,$rev3))"
+
 Hooks.
 ======