PHASING

In music the compositional technique 'phasing', popularized by composer Steve Reich, the same part is played on two musical instruments, one instrumentalist keeps playing in steady tempo, while the other gradually moves ahead of the first until it becomes out of and then back in phase (the rhythmic equivalent of cycling through the phase of two waveforms as in phasing). Note that both parts are still perceived as being in the same tempo with the change being only enough to gradually separate the parts and bring them back together. In some cases, especially live performance where gradual separation is extremely difficult, phasing is accomplished by periodically inserting an extra note into the phrase of one of the two players who are playing the same repeated phrase, thus shifting the phase by a single beat at a time, rather than gradually.
The technique originated in Reich's tape music, where the composer sets off several copies of the same tape loop simultaneously on different machines. Over time, the slight differences in the speed of the different tape machines causes a flanging effect and then rhythmic separation to occur. Examples include Reich's ''Come Out'' and ''It's Gonna Rain''. This technique was then extended to acoustic instruments as described in the above paragraph and later the change in phase was made immediate, rather than gradual, as in Reich's ''Clapping Music''.
An example in popular music is "The True Wheel" on Brian Eno's ''Taking Tiger Mountain By Strategy (album)''.
The playing of different repeated phrases in the same tempo but having different metrical lengths (beats in the bar), as in the music of Philip Glass and others, is not phasing but may be considered polyrhythms.