Moto perpetuo (perpetual motion) is really a "study” not only of certain technical skills for the player but also a study in writing a piece that is almost entirely diatonic (traditional major and minor) but with shifts in key and harmony that are "non-functional," that is, often changing from one key to another that is not closely related to it. The piece doesn't really feature a melody in the normal sense, but is built from arpeggios (broken chords) and scale patterns. From the technical standpoint, the dominant element is running eighth notes played in the manual (hands) with slightly slower quarter notes played in the pedals, often with pedal notes both above and below the notes played in the hands, so it's an unusual coordination that's needed. While the piece moves along in 2/2 time the rapid notes often shift to cross-rhythms, where the eighth notes are grouped in 3's; thus, the pedal part doesn't consistently align with what is happening above it.

The harmony is the most unique element. Often that harmony moves up and down by major thirds, but this is accomplished smoothly by use of notes that are in common between the chords involved, with other notes moving just a semitone or tone. To take an example from early in the piece, measures 11–14 outline an inversion of an E♭ minor 9th chord (E♭, G♭, B♭, D♭, and F). In measure 15, the harmony shifts to a B-minor 11th chord (B, D♮, F♯, C♯, F♯). This happens smoothly because there are two notes in common between those two rather rich chords: F♯ (enharmonic of G♭), C♯ (enharmonic of D♭), and the other notes shift only a half-step (B♭ to B♮, E♭ to D, F to E♮), hence the ear is struck more by a change of color within almost the same pattern of notes, rather than the significant harmonic shift involved. There are brief passages using the octatonic scale, and another using Messiaen's sixth "mode of limited transposition" in measures 39–42 (the scale in full would be F, G, A, A♯, B, C♯, D♯, E, F). Through most of the piece, though, the figurations are minor or major, though sometimes even richer harmonies, such as 11th or 13th chords.

So, how would you explain this to the lay listener? Try this: Throughout the piece, the harmony changes in what might be surprising ways, but with some notes in common between any two chords, and other notes moving only slightly. What we perceive is a change in color of what seems to be almost the same pattern of notes.

In reality, the motion in the piece isn't entirely perpetual, honestly. The central part of the piece – about one third of the overall piece, is in a slower tempo, with the running eighth notes giving way to a mixture of slow and fast notes. Toward the end of that section, a figure of ascending notes starts to appear, just two notes at first, then gradually more and more until it has evolved into a scale. Those ascending figures also help to tie together the many shifts in harmony. The opening tempo and figurations return, but with different changes in harmony from what happened in the first section of the piece. The figuration evolves into ascending scales on the last page, neatly tying together elements of the opening of the piece with what went on in the middle section. All of this is done in a way that is both attractive and beguiling, certainly immediately appealing to the listener!

—The GCNA