While Reich's It's Gonna Rain (1965), Come Out (1966), and Melodica (1966) make use of identical tape loops moving gradually out of phase on separate tape players, Piano Phase signals Reich's first attempt to apply phasing techniques in live performance. Composed of short, repeated patterns and scored for two pianos (or, in an alternate version, for two marimbas), Piano Phase is a study in rhythmic phase shifting based on a minimum of melodic and timbral resources. Though austerely constructed, the subtle variations that result from the gradual movement of one musical line ahead of itself imbue Piano Phase with surprising complexity.
Upon completing a number of early phase-shifting compositions that involved short recorded fragments of speech and music, Reich sensed a challenge: to bring the phasing process out of the recording studio and into performance arenas. In late 1966 he recorded himself performing a short melodic pattern on the piano, made a tape loop of that recording, and began to play that same pattern along with the tape loop. Playing slightly faster than the tape and incrementally moving further and further ahead of the recorded pattern, Reich approximated the effect of electronically produced phasing. Though not machine-accurate, Reich found that a human performer could gradually move in and out of phase with a repeated pattern and, with this realization, began work on Piano Phase.
At the onset of the piece, both pianists play the same brief pattern, repeating it quickly, again and again. While one pianist continues to play the pattern at the originial tempo, the other player accelerates slightly, moving forward until he or she is one note ahead of the first. This process of repetition and acceleration continues throughout the composition as both pianists move in and out of phase with one another until a final unison is reached during the piece's closing moments.
The shifting, shimmering melodies of Piano Phase mark an important turn in Reich's work away from purely electronic pieces and toward a new approach to counterpoint and performance; similar themes reappears in Violin Phase (1967), Drumming (1971), and many other of Reich's subsequent offerings.