Phasing is a compositional technique in which the same part (a repetitive phrase) is played on two musical instruments, in steady but not identical tempo. 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 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 waveforms as in phasing. Note that the tempi of the two instruments are almost identical, so that both parts are perceived as being in the same tempo: the changes only separate the parts gradually. In some cases, especially live performance where gradual separation is extremely difficult, phasing is accomplished by periodically inserting an extra note (or temporarily removing one) into the phrase of one of the two players playing the same repeated phrase, thus shifting the phase by a single beat at a time, rather than gradually.
It was popularized by composer Steve Reich. 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 in his Piano Phase, Reich's first attempt at applying the phasing technique to live performance, and later the change in phase was made immediate, rather than gradual, as in Reich's Clapping Music.
As the cycle unfolds, often other melodies will be created by the differing instances of the original phrase being played together. As in Steve Reich's Violin Phase, the composer will sometimes have an additional instrument or live performer with the tape who playing these secondary, extracted melodies to accentuate them.
Music writer Kyle Gann has pointed out on later use of phase shifting technique: "Though not widely used in minimalist works per se, it survived as an important archetype in postminimal music (e.g. William Duckworth's The Time Curve Preludes, John Luther Adams's Dream in White on White, Kyle Gann's Time Does Not Exist)."  Petri Kuljuntausta's Violin Tone Orchestra, Words and When I Am Laid In Earth are all phasing works and composed with the help of digital sampler, or computer, and multitrack technology. His electronic multichannel music for video installation Eight Rooms is based on spatial phasing, where the phase shifted sounds surrounds the listeners and moves around them. Kuljuntausta's phase shifting method is based on Nano Tuning of sounds.
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 polymeters.
The effect is similar to that heard when a Shortwave station undergoes fading. As the signal takes multiple paths through the ionosphere, the different time delays cause the signal to exhibit the characteristic phasing sound.
- The Time Curve Preludes streamed for free from Irritable Hedgehog Music
- Rhythm Phasing (Video Resources) - more examples made with Bounce Metronome Pro - a program which can apply phasing to rhythms.
- Peter Aidu plays Steve Reich's "Piano Phase"