one of a group of music theoretical abstractions that seek to capture and codify the main rhythmic patterns of French (primarily Parisian) polyphony of the late 12th and 13th centuries. These patterns are observable in the simplest pieces of the time and in individual segments thereof, whether organum, clausula, conductus, or motet, although the system does not always apply to more complex works.

Medieval theorists did not fully agree on how many patterns were to be classified or how they were to be presented. Most, however, wrote in terms of six patterns that may be viewed as analogous to the simpler poetic metres (metre)—I (trochee), II (iamb), III (dactyl), IV (anapest), V (spondee), and VI (tribrach). The early notation of the time grouped individual pitches within compound symbols known as ligatures, and the intended rhythms were indicated by standardized ligature patterns rather than by individualized note shapes. The earliest terminology for rhythmic values, the long (longa) and breve (brevis), was most likely derived from the vocabulary of metrics. (For more on ligature notation in the context of music history, see musical notation: Evolution of Western staff notation (musical notation).)

During the 12th century, the tempo of most of the notated music was rapid enough that a long followed by a breve combined to form the basic pulse, which in turn had ternary subdivisions. These basic pulses were generally grouped in twos. By the end of the 13th century, the tempo had slowed to the point that the long and breve together were equivalent to three pulses, with a resultant ternary metre. More complex rhythmic patterns were developing in the music, and the notation reached the limit of its usefulness. By the mid-13th century, individual symbols were devised for as many as four time values; these eventually provided the basis for more flexible, varied rhythmic notation and laid the foundation for the modern system.