Polyrhythm

Polyrhythm is the simultaneous use of two or more conflicting rhythms, that are not readily perceived as deriving from one another, or as simple manifestations of the same meter. The rhythmic conflict may be the basis of an entire piece of music (cross-rhythm), or a momentary disruption. Polyrhythms can be distinguished from irrational rhythms, which can occur within the context of a single part; polyrhythms require at least two rhythms to be played concurrently, one of which is typically an irrational rhythm.

In western art music

In some European art music, polyrhythm periodically contradicts the prevailing meter. For example, polyrhythm is heard in the first few minutes of Beethoven's Third Symphony and in the first movement of Brahms's Violin Concerto.

Hemiola

Concerning the use of a two-over-three (2:3) hemiola in Beethoven's Sixth String Quartet, Ernest Walker states, "The vigorously effective Scherzo is in 3/4 time, but with a curiously persistent cross-rhythm that does its best to persuade us that it is really in 6/8."