Polyrhythm
Two different subdivisions of the same time span, superimposed.
- 3 against 2: in one beat, three notes (triplet) against two (eighth notes). Least common multiple 6: the triplet falls on points 1, 3, 5; the eighth notes on 1 and 4.
- 4 against 3: in a 3/4 bar, four equal notes (each worth three sixteenth notes, i.e. a dotted eighth) against the three beats. Least common multiple 12.
- In trap, triplet hi-hats over a sixteenth-note grid create a momentary 3 against 4.
Hemiola
Two bars of 3/4 felt as one bar of 3/2 (or vice versa): the accents group in twos instead of threes. In the Baroque it often appears at the cadence, to slow down the perception of tempo.
Polymeter and cross-rhythm
A pattern of a different length from the bar: a figure of 3 sixteenth notes repeated over a 16-sixteenth bar (x..x..x..x..x..x.) continually shifts against the 1 and lines up again after three bars. It is the basis of the “dotted” patterns of house, techno and future bass.
How to use it without confusing
- Keep a stable reference (four-on-the-floor kick or snare on 3).
- Declare the polyrhythm in a single layer (arpeggio, hi-hat, percussion).
- Resolve: bring everyone back into alignment at the start of the next section.
Example in A minor
Am arpeggio in figures of 3 sixteenth notes (A–C–E, A–C–E…) over a four-on-the-floor kick at 120 BPM: the arpeggio's accent falls each time at a different point of the bar.
Real-world example
“America” from Bernstein's West Side Story (1957) alternates bars of 6/8 and 3/4: the same duration divided once in two and once in three, hemiola turned into dance. Fela Kuti's afrobeat, by contrast, is an interweaving of polyrhythms among guitars, percussion and horns.