Time signature

The time signature (also known as meter, metre, and measure signature) is a convention in Western music notation to specify how many of a particular note value are contained in each measure (bar). The time signature is a notational device representing the meter, an auditory feature of the music. In a music score the time signature appears at the beginning as a time symbol or stacked numerals, such as or (read common time or four-four time, respectively), immediately following the key signature (or immediately following the clef symbol if the key signature is empty). A mid-score time signature, usually immediately following a barline, indicates a change of meter. Most time signatures are either simple (the note values are grouped in pairs) or compound (grouped in threes). Less-common signatures correspond to complex, mixed, additive, and irrational meters. Most time signatures consist of two numerals, one stacked above the other: The lower numeral indicates the note value that the signature is counting. This number is always a power of 2 (unless the time signature is irrational), usually 2, 4 or 8, but less often 16 is also used, usually in Baroque music. 2 corresponds to the half note (minim), 4 to the quarter note (crotchet), 8 to the eighth note (quaver), 16 to the sixteenth note (semiquaver). The upper numeral indicates how many such note values constitute a bar. For instance, means two quarter-notes (crotchets) per bar, while means four eighth-notes (quavers) per bar. The most common time signatures are , , and . By convention, two special symbols are sometimes used for and : The symbol is sometimes used for time, also called common time or imperfect time. The symbol is typically used in place of and is called alla breve or, colloquially, cut time or cut common time. These symbols derive from Mensural time signatures, described below. Simple meters are those whose upper number is 2, 3, or 4, sometimes described as duple meter, triple meter, and quadruple meter respectively.