In Western notation a sign or signs placed at the beginning of a composition, after the clef and any key signature, or in the course of a composition. It indicates the Metre of the piece or a change in metre for a part of the piece. In modern usage two figures are usually given, one above the other: the lower indicates the unit of measurement, relative to the semibreve; the upper indicates the number of units in each bar. Thus a signature of 3/2 indicates that there are three minims (‘half-notes’) in each bar; a signature 9/8 indicates that there are nine quavers (‘eighth-notes’) in each bar.
Some signatures are survivors of the system of proportions and mensuration signs (see Notation, §III, 3(vii), 4(iii): is used for 4/4 and for 2/2 (also called alla breve). is a relic of the medieval tempus imperfectum cum prolatione minore, a mensuration where each long contained two breves and each breve contained two semibreves. The sign is now used to indicate quick duple time, the beat falling on the minim rather than the crotchet. In medieval terms the tactus in time fell on the semibreve; in time it fell on the breve. A diminution of note values in the ratio 2:1 was thus indicated by the introduction of the signature, so that any note was subsequently worth only half its previous value. In the Middle Ages, as in modern usage, it occasionally obviated the need to write such notes as the fusa and semifusa (the modern semi- and demisemiquaver).
RICHARD RASTALL