Isorhythm: The Hidden Patterns

One of the defining characteristics of the late medieval period (and the ars nova, in particular) was the use of isorhythm. In short, isorhythm is the repetition of an arbitrary pattern of note durations in all or part of a musical composition. This repetition may occur in more than one voice, but each voice need not carry the same rhythm. In contrast to the rhythmic modes, isorhtyhm allows the vocal parts of a composition more freedom by, 1) allowing for a rhythm of arbitrary duration and structure and, 2) allowing for overlap between the rhythmic units of different voices. Isorhythm is essentially nonexistent in modern music, but it is instructive to consider a similar musical technique, the round.

Most of us are familiar with the campfire song, "Row, Row, Row Your Boat," in which one person starts singing the melody and then, after one or more measures are complete, another person begins singing the same melody. After each singer completes main melody line, they return to the beginning and repeat. This song structure could be considered a special kind of isorhythm because each singer repeats a rhythmic structure (that of the melody line) and the rhythms overlap -- that is, the singers finish the melody at different times. In medieval motets, a similar technique was used, but each singer would be singing a different melody and using a different rhythm. To generalize the previous example, imagine instead that, after the first singer completes a measure of the song, another singer jumps in singing only, "Row, row, row." This second singer repeats these three words over and over again, using a different set of pitches each time but speaking the words at the same rate. This three-word sequence can serve as the basic unit of isorhythm and is perhaps a closer approximation to how medieval composers would craft the lower voices of a motet.

Despite this basic understanding of isorhythm, the technique has so far eluded me in the pieces I've listened to. Unlike "Row, Row, Row Your Boat," medieval motets are highly complex and isorhythm was generally only used in some of the voices and for only part of a piece. Two of the most prominent practitioners of isorhythm were Philippe de Vitry and Guillaume de Machaut, but their pieces were composed to blend into a graceful whole, hiding the underlying structure. It's likely that the "undulating" feeling I get from Machaut's compositions is in part due to his use of isorhythm, but seldom can I pinpoint its origin. For the casual listener, I would recommend against trying too hard to find it -- their motets are better experienced as a unified whole.