Devised in the 12th century, they used stereotyped combinations of ligatures (joined noteheads) to indicate the patterns of long notes (longs) and short notes (breves), enabling a performer to recognize which of the six rhythmic modes was intended for a given passage. The reading and performance of the music notated using the rhythmic modes was thus based on context. After recognizing which of the six modes applied to a passage of neumes, a singer would generally continue on in that same mode until the end of a phrase, or a cadence. In moderni editions of medieval music, ligatures are represented by horizontal brackets over the notes contained within it.
Though the use of the rhythmic modes is the most characteristic feature of the music of the Notre Dame school, especially the compositions of Léonin and Pérotin, they are also predominant in much of the rest of the music of the ars antiqua through about the end of the 13th century. Composition types which were permeated by the modal rhythm include Notre Dame organum (most famously, the organum triplum and organum quadruplum of Pérotin), conductus, and discant clausulae. Later in the century, the motets by Petrus de Cruce and the many anonymous composers, which were descended from discant clausulae, also used modal rhythm, often with much greater complexity than was found earlier in the century: for example each voice sometimes sang in a different mode, as well as a different language.
There were six rhythmic modes, as first explained in the anonymous treatise of about 1240, De mensurabili musica (formerly attributed to Johannes de Garlandia, who is now known to have edited it extensively in the late 13th century). Each mode consisted of a short grouping of long and short note values ("longs" and "breves") corresponding to a metrical foot, as follows:
Although this system of six modes was recognized by medieval theorists, in practice only the first three appear to have been the standard patterns of the modal period, with the second mode being less frequently used than the first and third. The fourth mode is almost never seen, and the fifth and sixth are hardly modes at all in view of their uniform rhythm.
Modern transcriptions of the six modes usually are as follows:
All the modes adhere to a ternary principle of metre, meaning that each mode would have a number of beats divisible by the number 3. This is speculated to have its origins in theology where the trinity was perfection and, as all music strives for perfection, it therefore had to be divisible by the number 3. Less speculatively, the flexibility of rhythm possible within the system allows for variety and avoids monotony. Notes could be broken down into shorter units or two notes of the same mode could be combined into one.
An ordo (plural ordines) is a phrase constructed from one or more statements of one modal pattern and ending in a rest. Ordines were described according to the number of repetitions and the position of the concluding rest. "Perfect" ordines ended with the first note of the pattern followed by a rest substituting for the second half of the pattern, and "imperfect" ordines ended in the last note of the pattern followed by a rest equal to the first part. Imperfect ordines are mostly theoretical and rare in practice where perfect ordines predominate. (ibid, p.223)
Other writers who covered the topic of rhythmic modes include Anonymous IV, who mentions the names of the composers Léonin and Pérotin as well as some of their major works, and Franco of Cologne, who recognized the limitations of the system and was the first to propose a notational method whereby the duration of any note would be signified by its shape. After Garlandia, many theorists tried to include the semibreve in the system. Lambertus increased the number of modes to 9.