+Footnotes in music expressions fall into two categories:
+
+@table @emph
+@item Event-based footnotes
+are attached to a particular event. Examples
+for such events are single notes, notes inside a chord, articulations
+(like beams, slurs, fingering indications, accents, dynamics) and
+lyrics.
+
+@item Time-based footnotes
+are bound to a particular point of time in a
+musical context. Some commands like @code{\time} and @code{\clef}
+don't actually use events for creating objects like time signatures
+and clefs. Neither does a chord create an event of its own: its
+stem or flag is created at the end of a time step (nominally through
+one of the note events inside). A time-based footnote allows
+annotating such layout objects without referring to an event.
+
+@end table
+
+The full form of a footnote command for both Event- and Time-based
+footnotes is