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In medieval music, the rhythmic modes were patterns of long and short durations (or rhythms) imposed on written notes which otherwise appeared to be identical. Modal notation was developed by the composers of the Notre Dame School, 1170 to 1250, replaced the even and unmeasured rhythm of early polyphony and plainchant with patterns based on the metric feet of classical poetry, and was the first step towards the development of modern mensural notation. (Hoppin 1978, p.221)

The reading and performance of the rhythmic modes was based on context; a singer would know which pattern of long and short to apply to a string of notes from the position of those notes within a phrase, and from an introductory notation at the start of the phrase indicating a mode number. 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. Modes were identifiable through the use of ligatures, which grouped together neumes into clusters. A performer would then be able to recognize which of the six rhythmic modes was intended for a given passage.

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:

1. Long-short (trochee)
2. Short-long (iamb)
3. Long-short-short (dactyl)
4. Short-short-long (anapest)
5. Long-long (spondee)
6. Short-short (pyrrhic)

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:

1. Quarter (crotchet), eighth (quaver) (generally barred, therefore, in 3/8)
   
2. Eighth, quarter (barred in 3/8)
   
3. Dotted quarter, eighth, quarter (barred in 6/8)
   
4. Eighth, quarter, dotted quarter (barred in 6/8)
   
5. Dotted quarters (barred in either 3/8 or 6/8)
   
6. Eighths (barred in 3/8 or 6/8)
   

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 is the principal source of the music of Léonin and Pérotin, 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.

• Articles "Rhythmic mode", "Johannes de Garlandia," "Franco of Cologne," in The New Grove Dictionary of Music and Musicians, ed. Stanley Sadie. 20 vol. London, Macmillan Publishers Ltd., 1980. ISBN 1-56159-174-2.
• The New Harvard Dictionary of Music, ed. Don Randel. Cambridge, Massachusetts, Harvard University Press, 1986. ISBN 0-674-61525-5.
• Richard H. Hoppin, Medieval Music. New York, W.W. Norton & Co., 1978. ISBN 0-393-09090-6.
• Parrish, Carl, The Notation of Medieval Music. London, Faber & Faber, 1957.