Two voices play the same five-note pattern. Voice A is locked at 120 BPM. Voice B drifts ahead.
Steve Reich found this in 1965 with two tape loops of the same recording on two slightly mistuned reels. The drift IS the composition.
PLAY to start. Drag the slider to change Voice B's tempo. The phase angle wraps from 0° back to 0° every time they realign.