No matter how difficult it is to define Minimalism, it is undeniable that it is a very exciting time to be listening to and thinking about it. The very actions we take, listening to the music, buying CDs, writing about the music, reading about the composers, are still changing the history of the movement. All of the comfortable labels we have for historical movements were coined long after they were over, and it is possible that the greatest Minimalist composer is someone we’ve never heard of. Furthermore, there is great danger in establishing a narrative. It can cause us to reject composers, and their music for fear of upsetting a comfortable set of ideas that we have become invested in.
If there is a Minimalist canon, however, Piano Phase is in it. I want to look at three different facets of Steve Reich’s 1967 composition Piano Phase and explain how  Piano Phase’s roots in mechanical delay opened the door for experimentation in the difference between sound production and the final acoustic product, it pioneered the idea of musical process as iterated function, and it was the first acoustic work with shifting, yet not cleanly divided, sections. This particular composition contains the ideas that Reich has expanded upon in his instrumental music, as well as provided a new theoretical framework for experimental music.
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The construction of Piano Phase requires us to examine the difference between the sound emitted by a performer or record reproducer and the sound perceived as music by the listener. It’s also important to look at an earlier Reich sound composition that, through mechanical means, showed Reich a different way to process music than the Western Classical techniques of development. That piece was Come Out.
From Wikipedia:

Come Out is a 1966 piece by American composer Steve Reich. Reich was asked to write this piece to be performed at a benefit for the retrial of the Harlem Six, six black youths arrested for committing a murder during the 1964 Harlem riots for which only one of the six was responsible. Truman Nelson, a civil rights activist and the person who had asked Reich to compose the piece, gave him a collection of tapes with recorded voices to use as source material. Nelson, who chose Reich on the basis of his earlier work It’s Gonna Rain, agreed to give him creative freedom for the project.

What Reich physically did with those tape loops was to duplicate them so that he had two copies of the same loop. Then he would play them simultaneously on two different tape players. The small difference in timing between those machines would create a delay effect. By taping that resultant and repeating the process many more times, or offsetting them by some fraction of time, a complexity builds.
The phasing process contained in Piano Phase is a direct attempt to emulate with live performers the mechanical delay effect used in Reich’s Come Out.  In Reich’s words, “Looking back on the tape pieces that preceded Piano Phase, I see that they were, on the one hand, realizations of an idea that was indigenous to machines and, on the other hand, the gateway to some instrumental music that I would never have come to by listening to any other Western, or for that matter non-Western, music.” It is important to understand the details of that delay process. The musical “material” is the electromagnetic information stored on the tapes, however the “music” only comes together when that material is played at the same time on two machines. The machines are not capable of changing the material. In this way, the material is both completely controlled and completely predetermined. This is also true of Piano Phase. All of the musical information is contained in the first bar of the score. To fully notate the polyrhythms that arise as the second piano accelerates in tempo would not only be nearly impossible, it would be incorrect. It might be an accurate notation of what the listener is hearing, but it would be a misrepresentation of the musical material. If this sounds farfetched, there is precedent in Western music (Reich notwithstanding). In the Italian Baroque, there were many experiments with polychoral music, music for multiple choirs singing at the same time. As with phasing, it would be inaccurate to call it choral music in eight parts; the music is not only the sound produced by the singers but the choirs’ positions relative to the listener and the way the overlapping sound waves interact. Let’s look  at that interaction in Piano Phase itself.
A helpful metaphor for the phasing process is that of a mathematical function. In the case of Piano Phase, that function is for the second piano to increase tempo gradually until it has moved ahead by one sixteenth-note. Then that result is subject to another iteration of the function. Other composers had been experimenting with compositions defined by a process, but Piano Phase is distinctive because it takes the output of a process and uses it as the input for another iteration of that same function. In the same way that the first measure of the score contains all of the melodic material of the piece, the complete function has been run once the second piano is offset from the first by a sixteenth-note. This is different from the process that governs Terry Riley’s In C; that composition has a length equal to the time it takes to execute its function once. What Reich has created is a new system of development. He has designed this function so that, with all note values equal, the pianos return to unison once the function has been run a number of times equal to the number of notes in the material.
Perhaps the most radical musical innovation of Piano Phase is the way that the phasing process completely eliminates the division between sections. This is not to say that there are no sections, it is clearly audible when the second piano “snaps” into place playing note against note with the first piano, but that the moment when one rhythm or melodic fragment recedes into the background and another dominates is completely subjective and is not found anywhere in the music. The 12 notes that make up the piece arranged such that, depending on how the pianos are offset, the listener can perceive an infinite number of “melodies.” This is also true of the rhythms; each listener subjectively arranges what they are hearing into rhythmic groupings. This is a technique that Reich uses over and over again in his instrumental work, from Music for 18 Musicians to his recent Double Sextet. One might object to my use of the term “radical;” this is also true of In C (which Reich helped premiere three years before composing Piano Phase). However, the choice that Riley gives his performers makes all the difference. Again Reich, “Everything is worked out, there is no improvisation whatever.” A thousand perfect performances of Piano Phase are going to be exactly the same. If the listener supplies the rhythmic and melodic hierarchies, and yet everything is worked out, then Reich has managed to eliminate them completely from the work itself. This represents no less a break with the course of Western music than the move away from tonality.
Piano Phase might not be the great Minimalist masterpiece, however it is essential to understanding Reich’s compositional style. It also puts to rest the idea that Reich’s tape loop compositions are anomalies from an early experimental phase. Rather, Piano Phase is a direct descendent of those experiments, and he has been refining these techniques throughout his career.