The undertone series is a subharmonic sequence of partials that inverts the intervals of the overtone series. In the undertone series, the partials go down from the fundamental rather than up. With the undertone series, more and more partials are clustered into the lower registers, where they are more indistinct. In this work, there are many tones that originate one to two octaves above the highest note on the piano. For notes in the extreme high range, the piece uses only “upper” partials (which are actually lower), and notes in the lower range use only “lower” partials (which are in the higher range).