The composition typically lasts around 15 minutes. The section begins by both pianists playing a rapid twelve-note melodic figure over and over again in unison (E4 F4 B4 C5 D5 F4 E4 C5 B4 F4 D5 C5). The pattern consists only of 5 distinct pitch classes. After a while, one pianist begins to play slightly faster than the other. When this pianist is playing the second note of the figure at the same time the other pianist is playing the first note, the two pianists play at the same tempo again. The process is repeated until the process has gone full circle, and the two pianists are playing in perfect unison. The second pianist then fades out, leaving the first playing the original twelve-note melody. The first pianist adjusts the bottom part to a four-note motif, which changes the pattern to an 8-note repeating pattern. The second pianist re-enters, but with a distinct 8-note pattern. The phasing process begins again; after the full eight cycles, the first pianist fades out, leaving one eight-note melody playing. The section ends at measure 26. The last section introduces the simplest pattern, now in 4/8 meter, built from final four notes of the melody from the previous section, and having only four distinct pitch classes. The other pianist re-enters, the phasing process restarts, and ends when both pianists return to unison. The phase cycle is repeated "ad libitum" from eight to sixty times according to the score. "Piano Phase" is an example of "music as a gradual process," as Reich stated in his essay from 1968. In it, Reich described his interest in using processes to generate music, particularly noting how the process is perceived by the listener. (Processes are deterministic: a description of the process can describe an entire whole composition. In other words, once the basic pattern and the phase process have been defined, the music consists itself.) Reich called the unexpected ways change occurred via the process "by-products", formed by the superimposition of patterns. The superimpositions form sub-melodies, often spontaneously due to echo, resonance, dynamics, and tempo, and the general perception of the listener. According to musicologist Keith Potter, "Piano Phase" led to several breakthroughs that would mark Reich's future compositions. The first is the discovery of using simple but flexible harmonic material, which produces remarkable musical results when phasing occurs.