time signature

In musical notation, a numerical sign placed after the clef and key signature indicating the metre of the music. Consisting usually of two numbers, the upper number represents the number of beats in a bar, the lower number the type of beat, expressed as a fraction of a unit (semibreve). Hence 3/4 is three crotchet beats to the bar and 6/8 is two beats each of three quavers; alla breve represents 2/2, or two minim beats to a bar; C (common time) represents 4/4, four crotchet beats to a bar.

Originally dictating a simple metre for a whole piece, since the late 19th century time signatures have become increasingly complex, with many 20th century composers frequently changing time signatures within a piece, and using previously unheard of metres (such as 11/8 or 5/16) to notate complex rhythms.