What is Isorhythm?

Isorhythm is a compositional technique developed in the XIII century and in use up to the XV century. It involves repetitive use of rhythmic patterns (prefix iso of Greek origin means equal).

In the Middle Ages it was common for a composer to use an existing melody and use it to build an original musical work. This melody becomes a tenor that serves as the foundation to one or more original melodic lines. The tenor below was used to create an anonymous motet:

The tenor is based on a Gregorian chant:

The author of the motet follows the melodic line of Gregorian chant but adds a rhythmic pattern that repeats throughout the tenor with the exception of the closing measures:

The rhythmic pattern is called talea. From bar 13, the composer repeats the same notes of the Gregorian chant in bars 1 to 12. This melodic pattern is called color. The tenor is created using the melodic pattern or color and the rhythmic pattern or talea. It is worth noting that in this case the color is repeated only twice while the talea, much shorter, is repeated eleven times. In the first example we mark the taleas with solid line and colors with a dashed line.

This compositional technique is called isorhythm and was developed by medieval composers. It will be widely used by the French composer Guillaume de Machaut as we shall see in the analysis of the Agnus from his Messe de Nostre Dame.