Most of the music Terry Riley composes is not written down. In C was an early exception, and remained one of his few notated pieces until Kronos Quartet began commissioning him in the 1980s. Riley specifically scored In C so that it would be open to many interpretations. The score is so minimal it fits on a single page, and Riley felt, "if you can't do it with just that, it's not worth doing".
The score of In C consists of 53 modules that fit on a single page. Each module is a short musical phrase notated in treble clef without a time signature and bracketed by repeat signs. Riley uses nine different pitches, only omitting C♯ and E♠from the chromatic scale.[21]: 49 
The total duration of the written score is only 521 eighth notes. The shortest module lasts one 8th note, and the longest lasts 64. The material varies widely in character, from drones to running 16th note figures. Three of the modules are repeated: Nos. 10 (as 41), 11 (as 36), and 18 (as 28). The longest figure is #35, which spans 60 pulses, ranges an octave and a half, and includes seven of the score's pitches. Its length creates a sense of figure 35 as a turning point in the piece, creating a symmetry or even hinting at a very loose ternary form.[21]: 52 
Riley indicates no tempo, instrumentation, or dynamics. The notation is extremely uncluttered, and by implication, open to a wide range of interpretation. The first melody outlines a major third with its three quarter note Es ornamented with grace note Cs. The final melody is a minor third between G and Bâ™ played in sixteenth notes. The structure of the melodic modules creates a vague sense of tonal shifts, for instance from C to E and then C to G.