(from Gr. isos, equal).
Term coined in 1904 by F. Ludwig to describe the principle found in medieval mus., c. 1300–1450, whereby the same rhythmic pattern recurs in successive repetitions of the melody. It was usually applied to the ten. part of a motet which would consist of a short repeated rhythmic pattern; the melody in the ten. part was also often repeated but not in synchronization with the rhythmic repetition. Rhythmic repetition was known as talea, melodic as color. In the mus. of Machaut and de Vitry, the rhythmic pattern was not repeated in the same note values but in proportional diminution, e.g. original values were halved or reduced by a third. Eng. composers at the end of the 14th cent. developed isorhythm in all vv., so that it took two appearances of the talea to accommodate one of the color.