close of the first section as the attainment of the e–c missing from the E–G of bars 13–17. However, the B which divides G–D require an F, "if we assume that the G–B–D collection represents an incomplete diminished-seventh chord, that the work is based on structural relations derived from the interval-3 cycle, and that this implies a tendency for each partition of the cycle to be completely represented". From the second beat of bar 56 until the end, however, the piece uses an interval-2 cycle, C2, which still produces tritones, but partitions them differently. E–B is paired with D–(G) and C–F, rather than C–G. The only odd interval (outside the interval-2 cycle) is the F–C, "which transfers us from C20 to C21. And here, in the last two notes of Density 21.5 we at last find B paired with its tritone associate ... only as a consequence of an entirely new harmonic direction that the work takes in its closing bars." However, "the music of the 'common practice' offers many exemplars of such altogether unexpected digressions just as a work is drawing to its close, followed by a return, for the final cadence, to a consequently more emphatic confirmation of the structural relations implied in the body of the work." In the foreground, the head motif, presumably Varèse's atonal "idea", does not contain a minor third or tritone, and thus each pitch, F–E–F, is a member of a different interval-3 cycle or diminished-seventh chord. The structural importance of each pitch is then dependent on its context, at which level it partitions the octave, the tritone which divides the octave, the minor third which divides the tritone, the major second which divides the major third, the minor second which divides the major second. "This variable relation of the basic motive of Density 21.5 to the harmonic structure of the piece, and its function in articulating and clarifying the formal design, are exactly what we would expect and take for granted in the relation between motive and background in traditional tonal music." Defined by the head motif and its repetition in bar 3, is "characterized by its relative pitch content (a three-note segment of the semitonal scale), by its interval order—down a half step and up a whole step—and by its rhythm (two sixteenth-notes on the beat followed by a tied eighth-note). This definitive version of the motive, combining all three attributes,