Sometimes a piece of music is all about the idea behind its construction. This notion goes all the way back to the earliest notated music we know about, and the early repertory of vocal music is full of arcane structural devices not apparent to the listener. One example of this sort of thing is the canon – a piece of music based on the idea of strict imitation. In it’s simplest form, it’s a round: “Row, row, row your boat” is a canon. One voice sings the idea and it is repeated exactly by the second voice. But you could make that into a retrograde canon by doing the second voice in reverse: “boat your row, row, row.” Composers love ‘em: every third variation in Bach’s famous “Goldbergs” is a two-voice canon in which the second voice starts on progressively higher scale degrees.

So here is a canon in the hands of Conlon Nancarrow, the wildly eccentric composer for player piano. In this case, this canon for two voice is all about time. Both voices are based on the same 54-note melody, but at the start the treble voice plays that melody ten times faster than the bass. As the piece progresses the notes in the treble slow down while the notes in the bass speed up in exact proportion. The time values cross in the middle (the “X” of the title). The overall effect is astonishing. And it was all done by hand, using paper and pencil to plot out the formulae and a piano roll punching machine to manually create the roll.