During the late '60s, Philip Glass undertook a series of pieces that together constitute a kind of "research and development" phase for his subsequent minimalist style. These pieces come across as extremely austere and focused, as if Glass were conducting a set of controlled experiments, each one having a distinct purpose. Composed in 1969, Music in Fifths is one of Glass' unrelentingly obsessive and procedural works, exploring as it does Glass' emerging "additive/subtractive" compositional technique within a rigidly limited collection of pitches that are organized in a very limited number of ways. The tight control of pitch materials, unchanging texture, and uninterrupted rhythmic flow draw the listener's attention almost exclusively to the expansions and contractions of line taking place within the static modal space. The work utilizes the same collection of pitches used in Glass' Two Pages, with one minor modification: instead of the wide-rising diad G to C, which was followed in Two Pages by a scalar ascent D to E flat to F, the G is shifted to the top of the collection to create a simple rising and falling scalar collection akin to a pianist's warm-up figure. As the title suggests, however, this contour is replicated below, beginning on the F and following in exact parallel motion. (The addition of lines moving together in this fashion would be explored later the same year by Glass in Music in Similar Motion). The full scalar figuration, consisting of simple up and down motion in balanced groups of four eighth notes, stands as the kind of "normative" melodic cell on which the piece is based. The piece does not begin with this figure, however, but rather eventually arrives at it before expanding beyond it. At the piece's opening, the general contour is presented in a kind of stilted version that rises along the scalar incline, but can't seem to find its way back down, resulting in a constant shift of metrical emphasis. Several different configurations of the ascending/descending gesture are attempted before finally arriving at the metrically stable and modally filled-out "normative" cell. Subsequently, each half of the cell is gradually augmented through the repetition of small constituent segments (two or three notes at a time), until the figure spans several measures. By the end of the work, the pitches and sonorities become nearly invisible while the unpredictable shifts of meter, direction, and contour become increasingly unsettling.

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