Twentieth-Century Rhythmic Techniques

Twentieth-century composers dramatically expanded the rhythmic vocabulary available to them, moving well beyond the regular metric patterns of tonal music. Four techniques appear most frequently in analysis: additive rhythm, polymeter, polyrhythm, and metric modulation. Each loosens the grip of a single steady pulse in a different way.

Additive rhythm builds measures or phrases by concatenating small, unequal groups — for example, 2+3+2 eighth notes — rather than dividing an even beat into equal subdivisions. Stravinsky's "The Rite of Spring" is the landmark example: bar lines fall wherever a new group begins, so the listener feels the music as a series of unequal impulses rather than a regular meter. Composers from Bartók to Messiaen extended this principle, sometimes encoding it in asymmetric time signatures such as 5/8 or 7/16.

Polymeter occurs when two or more layers of music proceed simultaneously in different meters — for instance, one instrument in 3/4 while another is in 4/4 — so the bar lines of the two layers rarely coincide. Polyrhythm, by contrast, keeps a shared beat but divides it into conflicting subdivisions at the same time, such as two against three or four against five. Both techniques create rhythmic tension through cross-accents that constantly shift the sense of where the beat falls.

Metric modulation (also called tempo modulation) is a precise technique in which a composer redefines the pulse unit at a structural boundary: a subdivision of the old meter becomes the new beat, producing an exact, calculable change of tempo. Elliott Carter systematized it as a compositional language. Finally, ametric music — found in Ligeti's micropolyphony or certain works of Feldman — abandons a measurable pulse entirely, creating a temporal flow determined by duration and texture rather than counted beats.