Isorhythm
Medieval compositional technique in which a rhythmic pattern is repeated independently of the melody (esp. 14th-century motets).
What it means
Isorhythm, Greek for “same rhythm,” is a medieval compositional technique in which a fixed rhythmic pattern is repeated several times, independently of which notes sound at the time. Rhythm and melody are thus two separate, overlapping layers of order.
Musical use
The technique marks above all the tenor part of French motets of the 14th century, the era of the ars nova. The rhythmic pattern, the talea, is repeated several times, while the underlying melody, the color, can have a different length and repeat independently, so that talea and color coincide anew with each repetition. Composers such as Guillaume de Machaut used isorhythm to build complex, internally coherent forms.
Isorhythm should be distinguished from the simple ostinato, in which a pattern is usually repeated unchanged and without the complex overlap of talea and color, and from the canon, which repeats the same melody offset in time across several voices. Isorhythm concerns specifically the repetition of rhythm, not necessarily of melody.
Performance practice
When listening to isorhythmic music, first follow the basic rhythmic pattern in the tenor part and count its repetitions. Then compare how the melody above it combines new notes each time with the same rhythm, to recognise the interplay of talea and color.