Isorhythm

Isorhythm (Greek, “same rhythm”), technique of musical construction used particularly in the 14th-century motet. The term was coined early in the 20th century, although the Latin terms for its constituent parts, talea for a repeating rhythmic pattern, and color for a repeating melodic pattern, were used in medieval times. Isorhythm was one of the most important structural techniques of early music.

A repeating rhythmic pattern, known as a rhythmic mode, was used in the tenors (melodies derived from plainsong) of some 13th-century motets and clausulae (sections of plainsong with additional words, the ancestors of the motet). Gradually, in the 14th century, the tenor patterns grew longer, and were sometimes repeated according to exact proportional diminutions, such as two thirds or one third of the original note values. In a piece using the isorhythmic technique, a melodic pattern, the color, would also be repeated, though it was not necessarily of the same length as the talea. The talea was usually shorter, so that perhaps nine or ten would be stated in the same time as two or three colors. An overlapping effect could thus be created, as the talea started each time at a different point in the color.

Philippe de Vitry, the 14th-century French theorist and composer, is sometimes credited with the invention of isorhythm. In fact he standardized the procedure (typically with two statements of the plainsong tenor) and provided a model for his immediate followers. Ideally, the tenor talea would correspond to one stanza of each of the texts set in the two faster-moving upper voices, which also gradually became subject to isorhythm, particularly if there were a memorable figure such as a hocket (when two or more parts rhythmically interlock without overlapping). In many motets of the late 14th and early 15th centuries, isorhythm was extended to all parts, a technique sometimes known as pan-isorhythm. This was particularly prevalent with English composers of the late 14th and early 15th centuries, such as John Benet, Leonel Power, and John Dunstable.