In music notation, a sixty-fourth note (American), or hemidemisemiquaver or semidemisemiquaver (British), sometimes called a half-thirty-second note (Burrowes 1874, 42), is a note played for half the duration of a thirty-second note (or demisemiquaver), hence the name. It first occurs in the late 17th century and, apart from rare occurrences of hundred twenty-eighth notes (semihemidemisemiquavers) and two hundred fifty-sixth notes (demisemihemidemisemiquavers), it is the shortest value found in musical notation (Morehen 2001).
Sixty-fourth notes are notated with a filled-in oval note head and a straight note stem with four flags. The stem is drawn to the left of the note head going downward when the note is above or on the middle line of the staff. When the note head is below the middle line the stem is drawn to the right of the note head going upward. A single 64th note is always stemmed with flags, while two or more are usually beamed in groups (Gerou and Lusk 1996, p. 211).
A similar, but rarely encountered symbol is the sixty-fourth rest (or hemidemisemiquaver rest, shown on the right of the image) which denotes silence for the same duration as a sixty-fourth note.
Notes shorter than a sixty-fourth note are very rarely used, though the hundred twenty-eighth note--otherwise known as the semihemidemisemiquaver (Haas 2011, 112)--and even shorter notes, are occasionally found.