rhythm (in the full sense of the word) covers everything pertaining to the time aspect of mus. as distinct from the aspect of pitch, i.e. it incl. the effects of beats, accents, measures, grouping of notes into beats, grouping of beats into measures, grouping of measures into phrases, etc. When all these factors are judiciously treated by the performer (with due regularity yet with artistic purpose—an effect of forward movt.—and not mere machine-like accuracy) we feel and say that the performer possesses ‘a sense of rhythm’. There may be ‘free’ or ‘strict’ rhythm.

The human ear seems to demand the perceptible presence of a unit of time (the beat); even in the ‘free rhythm’ of plainsong this can be felt, though in such mus. the grouping into measures is not present.

Apart from such mus. as that just mentioned it will be found that the beats fall into regular groups of 2s or 3s, or of combinations of these (as a group of 4 made up of 2+2, or a group of 6 made up of 3+3). Such groups or combinations of groups are indicated in our notation by the drawing of bar-lines at regular intervals, so dividing the mus. into measures (or ‘bars’). The measures, in their turn, can be felt to build up into larger groups, or phrases (4 measures to a phrase being a very common but not invariable combination; cf. phrase).

It is chiefly accent that defines these groupings, e.g. taking the larger groupings, a 4-measure phrase is normally accentuated something like this:and if the beats are in any part of the music subdivided into what we may call shorter beat-units sub-accentuations are felt, as Where the measures have 3 beats an accented note is followed by 2 unaccented:and similarly in a 3-measure phrase the first measure will be more heavily accentuated than the 2 following measuresIt will be seen, then, that what we may call the official beat-unit of a composition is a convention, there being often present smaller units and always present larger units, both of which may be considered beats. Another example of free rhythm may be seen in much of the choral mus. of the polyphonic period (madrigals, motets, etc.): these may be said (in literary terms) to be in ‘prose rhythm’, as opposed to the ‘verse rhythm’ of most tunes for marching and dancing.

Just as the traditional conception of tonality dissolved at the beginning of the 20th cent., so the organization of rhythm became more elaborate, irregular, and surprising. It can be divided into 2 categories: (1) metrical, with irregular groups of short units, (2) non-metrical, where there is no perceptible unit of measurement and no ‘traditional’ tempo. Metrical rhythms predominated at the start of the century, but the different uses possible are illustrated by the contrast between Schoenberg's works c.1908-15, where constantly changing tempi and freer use of changing time signatures make the rhythmic structure highly complex, and Stravinsky's of the same period, where there are similar constant changes of time signature but the irregularities are much more clearly defined. Syncopation has also invaded all types of mus. Although syncopated rhythm can be found in the earliest music, in the 20th cent. it has stemmed mainly from jazz.

Non-metrical rhythm can be discerned in Wagner and its possibilities were outlined by Busoni, who wrote of the tense silence between movts. being in itself mus. and more ‘elastic’ than sound. Messiaen in the late 1930s developed ‘ametrical’ rhythm and described in a treatise (1944) that the techniques he used were ‘augmented or diminished rhythms’, ‘retrograde’ rhythms, and ‘polyrhythm’. From 1940 composers such as Babbitt, Boulez, and Messiaen himself developed these tendencies, though some find the results ‘static’ rather than conveying the sense of impetus which is the function of rhythm. Further revolutionary attitudes to rhythm have developed since the 1950s, with the increasing use of indeterminacy. Composers such as Cage, Stockhausen, Carter, and Xenakis have written works which leave the choice of duration and tempo to the performer. With the introduction of elec. and scientific techniques into comp., there seems no limit to the expansion and intricacy of rhythmic procedures in mus.