-#(define-music-function (main grace) (ly:music? ly:music?)
- (_i "Create @var{grace} note(s) after a @var{main} music expression.")
+#(define-music-function (fraction main grace) ((fraction?) ly:music? ly:music?)
+ (_i "Create @var{grace} note(s) after a @var{main} music expression.
+
+The musical position of the grace expression is after a
+given fraction of the main note's duration has passed. If
+@var{fraction} is not specified as first argument, it is taken from
+@code{afterGraceFraction} which has a default value of @code{3/4}.")