Speed up a polyrhythm enough and it turns into a musical chord
Tap three even beats against two and you have a hemiola, the simplest polyrhythm. Speed the pattern up until the taps blur into tones and your ear hears a perfect fifth. Three against four becomes a perfect fourth, and four against five a major third, because the same ratios govern both rhythm and pitch.
A polyrhythm layers two or more rhythms that cannot easily be heard as parts of one shared meter. The underlying pulse itself can count as one layer, which is why the Cuban son clave qualifies: its three-stroke half implies a different meter from the pattern's overall pulse. When such layering runs through a whole piece rather than a passing moment, it is called cross-rhythm.
European composers have used polyrhythm to momentarily unsettle the beat. In Don Giovanni, Mozart has two onstage orchestras playing in different meters, later joined by a third. Chopin's Étude Op. 10 No. 10 sounds simple to listeners, but Alan Walker described it as a hornets' nest of cross-rhythms for the pianist. Brahms was so fond of layered rhythms that one writer likened them to an obsessive-compulsive streak in his music. Even the insistent four-note figure of Mykola Leontovych's Carol of the Bells is built from a two-against-three pattern.
In much of sub-Saharan Africa, cross-rhythm is not an occasional effect but the organising principle. Western rhythms tend to lean on the main beats, while African-derived rhythms stress the secondary ones, so newcomers often hear the background as foreground and mistake the true beat for the offbeat. The drummer C.K. Ladzekpo and the scholar David Locke were the first to explain cross-rhythm as the foundation of this music, and Eugene Novotney calls the three-against-two relationship the basis of most West African polyrhythmic textures.
Musicians there give it a philosophical reading too. Holding steady on the main beat while playing against it can stand for keeping one's purpose amid life's pressures. Many of these languages have no separate word for rhythm, or even for music, because rhythm is felt as part of the fabric of social life, an expression of how people depend on one another.
Source: Polyrhythm
