diff --git a/lisp/sort.el b/lisp/sort.el
index de0e1b9519..e22f062dbd 100644
--- a/lisp/sort.el
+++ b/lisp/sort.el
@@ -80,7 +80,15 @@ sort-subr
 PREDICATE, if non-nil, is the predicate function for comparing
 keys; it is called with two arguments, the keys to compare, and
 should return non-nil if the first key should sort before the
-second key.  If PREDICATE is nil, comparison is done with `<' if
+second key.  The key values PREDICATE is called with are the
+either the return values of STARTKEYFUN when that function is
+specified and returns a non-nil value.  In any other case the keys
+are cons cells of the form (BEG . END), where BEG is the value of
+point after calling STARTKEYFUN when given, else after calling
+ENDRECFUN, and END is the value of point after calling ENDKEYFUN when
+given, and after calling ENDRECFUN else.
+
+If PREDICATE is nil, comparison is done with `<' if
 the keys are numbers, with `compare-buffer-substrings' if the
 keys are cons cells (the car and cdr of each cons cell are taken
 as start and end positions), and with `string<' otherwise."