+Grace notes are ornaments are written out ornaments
+@lilypond[relative=2,fragment]
+ c4 \grace c16 c4 \grace { [c16 d16] } c4
+@end lilypond
+
+In normal notation, grace notes are supposed to take up no logical
+time in a measure. Such an idea is practical for normal notation, but
+is not strict enough to put it into a program. The model that LilyPond
+uses for grace notes internally is that all timing is done in two
+steps:
+
+Every point in musical time consists of two rational numbers: one
+denotes the logical time, one denotes the grace timing. The above
+example is shown here with timing tuples.
+
+@lilypond[relative=2,fragment]
+ c4^"(0,0)" \grace c16^"(1/4,-1/16)" c4^"(1/4,0)" \grace {
+ [c16^"(2/4,-1/8)" d16^"(2/4,-1/16)" ] } c4^"(2/4,0)"
+@end lilypond
+
+The advantage of this approach is that you can use almost any lilypond
+construction together with grace notes, for example slurs and clef
+changes may appear halfway in between grace notes:
+
+@lilypond[relative=2,fragment]
+ c4 \grace { [ c16 c, \clef bass c, b(] } )c4
+@end lilypond