Irrational rhythm

In music, an irrational rhythm is any rhythm in which an unnatural number of beats is played within the context of another rhythm prescribed by a time signature in which it does not subdivide exactly.

Unlike polyrhythms, which are two seperate rhythms played against one another, an irrational rhythm is an uncommon number of notes, pulses, or other rhythmic unit occuring in one beat in the context of a single rhythmic line.

The most familiar example is the triplet, in which three beats are played in the space of two. In compound time, the triplet can form the basic rhythmic unit (three triplets ³₈, three triplets-three triplets ⁶₈, and so on), and so a common irrational rhythm in compound time is the duplet.

Until the nineteenth century these were the only irrational rhythms that was commonly seen in written music; the Romantic composers then introduced the quintuplet, in which five beats are played in the space of four, creating a hurried, rushing effect. Such groupings are often written with figures of the form "5:4" above the notes; here the colon can be read off as "in the space of".

In many forms of modern classical music irrational rhythms have been greatly extended, with groupings such as 7:8 and even 11:8 or 11:16 appearing fairly commonly.