Rhythm in the Twentieth Century

Introduction: Time Set Free

Common-practice rhythm is built on regular meters: a steady beat, grouped in twos or threes, bar after bar. Twentieth-century composers kept the beat but loosened almost everything else. Meters change from bar to bar; beats come in unequal lengths; different layers run in different meters at once; tempos shift by exact proportions; and some music abandons meter entirely. Many of these ideas came from folk music (Bartók's Balkan dances), from jazz, and from non-Western traditions such as Indian tala and West African drumming.

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Changing Meters

Changing (or mixed) meters alter the time signature frequently, sometimes every bar. The effect is to keep the listener off balance and to let musical phrases dictate the bar lengths, rather than forcing phrases into a fixed meter. Stravinsky's The Rite of Spring (1913) is the classic example.

Asymmetric (Additive) Meters

An asymmetric meter divides each bar into beats of unequal length: groups of two and three eighth notes. They're also called additive meters because the bar is built by adding small units: 5/8 = 2 + 3 or 3 + 2; 7/8 = 2 + 2 + 3, 2 + 3 + 2, or 3 + 2 + 2. The grouping is shown by beaming (and sometimes in the time signature itself, as in 2+2+3/8). Such meters are common in Balkan and Greek folk music; Bartók's Six Dances in Bulgarian Rhythm and Brubeck's "Take Five" (5/4) made them famous in concert music and jazz.

Additive groupings can also appear inside a regular meter. The eight eighths of a 4/4 bar grouped 3 + 3 + 2 produce the tresillo rhythm heard in Latin American music, ragtime, rock, and pop.

Syncopation and Metric Displacement

Syncopation accents weak beats or offbeats against an established meter. When a pattern is consistently shifted off the beat, so that a whole layer seems to be in a displaced version of the meter, it's called metric displacement. A related effect is the hemiola: three groups of two in a space normally heard as two groups of three (6/8 heard briefly as 3/4), a device already common in Baroque cadences that Brahms used constantly.

Polyrhythm and Polymeter

Two terms describe layers that conflict:

• Polyrhythm: two (or more) divisions of the same span sound at once, e.g., three evenly spaced notes against two (3:2) or four against three (4:3). The bar lines coincide; the divisions don't.
• Polymeter: two layers in different meters with the same beat, so their bar lines fall in different places and realign only after several bars (e.g., 3/4 against 4/4 realigns every 12 beats).

Tuplets

Tuplets divide a beat into a number of parts the meter doesn't normally provide: triplets (3 in the time of 2), quintuplets (5 in the time of 4), septuplets (7 in the time of 4 or 8). Composers from Chopin to Messiaen to Ferneyhough use them to create flexible, rubato-like rhythm within a fixed beat.

Metric Modulation

Metric modulation (Elliott Carter's term) changes tempo by making a note value in the old tempo equal to a different note value in the new tempo. Because the shared value keeps its speed, the change is precise and smooth, much like a pivot chord in a key change. To calculate the new tempo:

1. Find how many of the shared value occur per minute in the old tempo.
2. That number is the speed of the equivalent value in the new tempo.
3. Convert to the new beat unit.

Messiaen and Rhythmic Process

• Added values: a short value (a dot, a note, or a rest) added to a regular rhythm to make it irregular.
• Nonretrogradable rhythms: rhythmic palindromes that sound the same forward and backward (e.g., ♪ ♩ ♩. ♩ ♪).
• Rhythmic augmentation and diminution, sometimes by unusual ratios (adding a dot, multiplying by 1.5).
• Isorhythm: a repeating rhythmic pattern (talea) combined with a repeating pitch pattern (color) of a different length, so they realign only after many cycles. Messiaen revived this medieval technique.
• Ametric music: no meter at all, notated without bar lines or in proportional notation (horizontal space = time), with feathered beams for accelerating or slowing notes.

Try It: Polyrhythm Player

Summary

• Changing meters alter the time signature frequently; asymmetric (additive) meters such as 5/8 and 7/8 have beats of unequal length (groups of 2 and 3).
• Syncopation, metric displacement, and hemiola play against the meter.
• Polyrhythm sets different divisions of one span against each other (3:2, 4:3); polymeter layers different meters on the same beat.
• Tuplets (quintuplets, septuplets…) divide beats irregularly.
• Metric modulation changes tempo by equating two different note values: compute the shared value's speed and convert.
• Messiaen's added values, nonretrogradable rhythms, and isorhythm, and ametric notation complete the toolkit.

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