Phase music is a form of music that uses phasing as a primary compositional process. It is an approach to musical composition that is often associated with minimal music, as it shares similar characteristics, but some commentators prefer to treat phase music as a separate category. Phasing is a compositional technique in which the same part (a repetitive phrase) is played on two musical instruments, in steady but not identical tempi. Thus, the two instruments gradually shift out of unison, creating first a slight echo as one instrument plays a little behind the other, then a doubling effect with each note heard twice, then a complex ringing effect, and eventually coming back through doubling and echo into unison. Phasing is the rhythmic equivalent of cycling through the phase of two wavefo