A measure has two purposes in Western traditions of music, the first is to block out a series of beatss, and the second is to form the building block of larger sections of music, such as a phrase. Time signatures imply strongly accented beats, and others that are less accented, changing time signature changes the pattern of emphasizing notes, either by playing certain notes louder, or by sustaining them as in swing or rubato. A measure is similar to a metrical foot in poetry (See Meter (poetry)).
There are four different types of metre in common use: simple duple (ex. 4/4), simple triple (ex. 3/4), compound duple (ex. 6/8), and compound triple (ex. 9/8). If each beat in a measure is divided into two parts, it is simple meter, and if divided into three it is compound. If each measure is divided into two beats, it is duple meter, and if three it is triple. Some people also label quadruple, while some consider it as two duples. The latter is more consistent with the above labeling system, as any other division above triple, such as quintuple, is considered as duple+triple (12123) or triple+duple (12312), depending on the accents in the musical example. However, in some music a quintuple may be treated and perceived as one unit of five, especially at faster tempos.
Most popular music is in 4/4 time, though often may be in 2/2 or cut time such as in bossa nova. Doo-wop and some other rock styles are frequently in 12/8, or may be interpreted as 4/4 with heavy swing. Similarly, most classical music before the 20th century tended to stick to relatively straightforward metres such as 4/4, 3/4 and 6/8, though variations on these such as 3/2 and 6/4 are also found. By the 20th century, composers were using less regular metres, such as 5/4 and 7/8.
Also in the 20th century, it became relatively more common to switch metre frequently—the end of Igor Stravinsky's The Rite of Spring is a particularly extreme example—and the use of asymmetrical rhythms where each beat is a different length became more common: such metres include already discussed quintuple rhythms as well as more complex constructs along the lines of 2+5+3/4 time, where each bar has a 2 beat unit, a five beat unit and a 3 beat unit, with a stress at the beginning of each unit—there are similar metres used in various folk musics. Other music has no meter at all (free time) such as drone based music exemplified by La Monte Young, feature rhythms so complex that any metre is obscured such as in serialism, or is based on additive rhythms, such as some music by Philip Glass.
Metre is often combined with a rhythmic pattern to produce a particular style. This is true of dance music, such as the waltz or tango, which have particular patterns of emphasizing beats which are instantly recognizable. This is often done to make the music coincide with slow or fast steps in the dance, and can be thought of as the musical equivalent of prosody.