Metre (music)

Metre or meter is the measurement of a musical line into measures of stressed and unstressed "beats", indicated in Western music notation by a symbol called a time signature. Properly, "metre" describes the whole concept of measuring rhythmic units, but it can also be used as a specific descriptor for a measurement of an individual piece as represented by the time signature—for example, "This piece is in 4/4 metre" is equivalent to "This piece is in 4/4 time" or "This piece has a 4/4 time signature".

Metre is an entrainment, a representation of changing aspects of music as patterns of temporal invariance, allowing listeners to synchronize their perception, cognition, and behaviour with musical rhythms. Rhythm is distinguished from metre in that rhythms are patterns of duration while "metre involves our initial perception as well as subsequent anticipation of a series of beats that we abstract from the rhythm surface of the music as it unfolds in time" (London 2004, p.4-5).

Ametric music includes chant, some graphically scored works since the 1950s, and non-European folk music such as honkyoku repertoire for shakuhachi (Karpinski 2000, p.19).

Rhythmic metre

There are four different time signatures in common use:
*Simple duple (ex. 4/4)
*Simple triple (ex. 3/4)
*Compound duple (ex. 6/8)
*Compound triple (ex. 9/8)

	Beats divided in two!Beats divided in three
Two beats per measure	simple duple	compound duple
Three beats per measure	simple triple	compound triple

If each beat in a measure is divided into two parts, it is simple metre, and if divided into three it is compound. If each measure is divided into two beats, it is duple metre, 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 labelling 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.

The piano intro to "Take Five", a jazz composition in 5/4, note that the rhythm suggests a 12312 division – Listen to this piece

"Once a metric hierarchy has been established, we, as listeners, will maintain that organization as long as minimal evidence is present" (Lester 1986, p.77). Duple time is far more common than triple (Krebs 2005, p.16). 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 5-beat unit, and a 3-beat unit, with a stress at the beginning of each unit; similar metres are used in various folk musics. Other music has no metre at all (free time) (such as drone-based music as exemplified by La Monte Young), features 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. Sometimes, a particular musician or composition becomes identified with a particular metric pattern; such is the case with the so-called Bo Diddley beat. Some examples (Scruton 1997):

March rhythms

Polka rhythms

Siciliano rhythms

Waltz rhythms

Polymetre

Polymetre is the use of two metric frameworks simultaneously, or in regular alternation. Examples include Béla Bartók's "Second String Quartet". Leonard Bernstein's "America" (from West Side Story) employs alternating measures of 6/8 (compound duple) and 3/4 (simple triple). This gives a strong sense of two, followed by three, stresses (indicated in bold type): // I-like-to be-in-A // ME RI CA//.

An example from the rock canon is "Kashmir" by the seminal British hard-rock quartet Led Zeppelin, in which the percussion articulates 4/4 while the melodic instruments present a riff in 3/4. In "Toads Of The Short Forest" (from the album Weasels Ripped My Flesh), composer Frank Zappa explains: "At this very moment on stage we have drummer A playing in 7/8, drummer B playing in 3/4, the bass playing in 3/4, the organ playing in 5/8, the tambourine playing in 3/4, and the alto sax blowing his nose." The math metal band Meshuggah uses complex polymetres even more extensively; typically the songs are constructed in 4/4, with guitar riffing and bass drum patterns in unusual metres such as 11/8 and 23/16. Usually the riffs are forced to resolve after 4 or 8 measures resulting in a shorter 'fitpiece' which has a different metre from the rest of the section.

Perceptually there appears to be little or no basis for polymetre as research shows that listeners either extract a composite pattern that is fitted to a metric framework, or focus on one rhythmic stream while treating others as "noise". This upholds the tenet that "the figure-ground dichotomy is fundamental to all perception" (Boring 1942, p.253), (London 2004, p.49-50).

Metric structure

Metric structure includes metre, tempo, and all rhythmic aspects which produce temporal regularity or structure, against which the foreground details or durational patterns are projected (Wittlich 1975, chap. 3).

Rhythmic units can be metric, intrametric, contrametric, or extrametric.

Metric levels may be distinguished. The beat level is the metric level at which pulses are heard as the basic time unit of the piece. Faster levels are division levels, and slower levels are multiple levels (Wittlich 1975, chap. 3).

Hypermetre is large-scale metre (as opposed to surface-level metre) created by hypermeasures which consist of hyperbeats (Stein 2005, p.329). The term was coined by Cone (1968) while London (2004, p.19) asserts that there is no perceptual distinction between metre and hypermetre.

A metric modulation is a modulation from one metric unit or metre to another.

Deep structure

C. S. Lee (1985) has described musical metre in terms of deep structure, where, through rewrite rules, different metres (4/4, 3/4, etc) generate many different surface rhythms. For example the first phrase of The Beatles' A Hard Day's Night, without the syncopation, may be generated from its metre of 4/4:

4/4 4/4 4/4

    /    \              /    \         /    \

2/4 2/4 2/4 2/4 2/4 2/4