A time signature indicates the meter in music. Meter denotes the division of a composition into units of equal time value. These units, or measures, usually stay consistent throughout a piece or movement of music. Some composers change the meter from measure to measure. A listener could focus on one rhythmic stream, bringing it to the forefront (figure) while hearing others as noise (background or ground).
Metric modulation is a change from one meter to another. In the twentieth century, it became more common to switch meter within a composition of music.
Polymeter is the use of two time signatures simultaneously. Polyrhythm refers to two or more different patterns which could be in the same time signature. An example of polymeter is the bass and drums playing in a 5/4 time signature, counting five beats per measure, while the vocals are in 4/4, counting only four beats per measure. Polymeter is also the use of two meters in regular alternation.
Additive rhythms are sometimes incorrectly called asymmetric rhythms. This irregular time signature may change from three to two and back to three beats in each measure while still in the 4/4 time signature. Additive meters may be long, irregular or constantly changing short meters.
The twentieth-century composer Bela Bartok took influences from the "asymmetric" rhythms in Bulgarian folk music. Composer Igor Stravinsky used changing meter in The Rite of Spring. Jazz musicians use metric modulation by changing to a double-time feel, while keeping the absolute length of a bar consistent.
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