Two Pages, for piano or keyboard, marks an important milestone in the development of Philip Glass' early compositional technique. Virtually all of his works completed during the late '60s demonstrate his interest in so-called "additive/subtractive" processes—that is, compositional procedures that expand a simple melodic cell into a large-scale structure by subjecting it to gradual expansion or contraction. Prior to Two Pages, Glass applied additive/subtractive melodic formulas in an intuitive fashion, lending his works unpredictable forms. Works composed in this manner include Strung Out (1967), a work for violin solo built upon a simple string of five tones; 600 Lines (1968), an ensemble work so unwieldy it was later dropped from Glass' works list; and 1+1, a somewhat conceptual work for two players' knuckles rapping on a table top, in which the composer provided only two rhythmic figures and the instructions to repeat and alternate between the figures according to an algorithm of the performers' own choosing. With Two Pages (1969), however, Glass introduced a more methodical approach to the additive/subtractive concept under development. The work is built upon a five-tone melodic cell G-C-D-E flat-F—identical to that of Strung Out, save with the lowest note of the earlier work transposed up an octave. In each of the piece's five sections, this fundamental melody is subjected to a different series of gradual transformations generated by a consistent formula. Part one expands upon the melodic cell by linking it to chains of sub-cells of successively decreasing size. Thus, at its peak, the cell is followed by its first four notes, then its first three, then two; these sub-cells are then gradually removed. Part two takes a similar form, but expands with sub-cells taken from the end of the figure to the point that the harmonic underpinning of the G-C leap is weakened by surprisingly long strings of D-E flat-F repetitions. Part three approaches from both directions, expanding the cell from both ends, while part four changes the sonority entirely by removing the low G altogether. The final section assumes a shape similar to that of the beginning. These seemingly minor changes to a rather static musical landscape have a surprising effect: by using repetition schemes that emphasize different pitches in the series, Glass slowly and subtly changes the harmonic hue suggested by the relentless and rapid-fire circulation of the limited pool of pitches, while at the same time the continual addition and subtraction of notes creates a fluid and unpredictable rhythmic and metric terrain. Aside from its importance as Glass' first methodical approach to additive/subtractive technique, it bears a certain historical and biographical importance as well. Once colleagues who performed each other's music, Philip Glass and his cohort Steve Reich later fell into dispute over issues of compositional preeminence—each questioning the priority of the other. Two Pages represents more amicable attitudes; although later shortened to its present two-word title, the piece was originally called "Two Pages for Steve Reich."

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