Additive rhythms are larger periods of time constructed from sequences of smaller rhythmic units added to the end of the previous unit. This is contrasted with divisive rhythms, in which a larger period of time is divided into smaller rhythmic units.

The relationship between additive and divisive rhythms is complex, and the terms are often used in imprecise ways. The seventh edition of the New Grove Dictionary of Music and Musicians, in its article on rhythm, states that "In discussions of rhythmic notation, practice or style, few terms are as confusing or as confusingly used as 'additive' and 'divisive'." Winold recommends that, "metric structure is best described through detailed analysis of pulse groupings on various levels rather than through attempts to represent the organization with a single term." (DeLone et. al. (Eds.), 1975, chap. 3)

Most western music is primarily divisive, while Indian and other musics, may be considered as primarily additive. However, most pieces of music cannot be clearly labeled divisive or additive. For instance, Ewe music uses additive rhythms against a time-background that is divisive.

The term additive rhythm is also often used to refer to what are also incorrectly called asymmetric rhythms and even irregular rhythms - that is, metres which have a regular pattern of beats of uneven length. For example, the time signature 4/4 indicates each bar is eight quavers long, and has four beats, each a crotchet (that is, two quavers) long. The asymmetric time signature 3+3+2/8, on the other hand, while also having eight quavers in a bar, divides them into three beats, the first three quavers long, the second three quavers long, and the last just two quavers long. These kinds of rhythms are used, for example, by Béla Bartók, who was influenced by similar rhtyhms in Bulgarian folk music, and in some music of Philip Glass, and other minimalists, most noticeably the "one_two_one_two_three" chorus parts in Einstein on the Beach. They may also occur in passing in pieces which are on the whole in conventional metres. Obviously the "asymmetric" rhythm 3+3+2 may be written 3+2+3, in which case it is symmetric, and if repeated regularly, no longer is it "irregular".

Source

• DeLone et. al. (Eds.) (1975). Aspects of Twentieth-Century Music. Englewood Cliffs, New Jersey: Prentice-Hall. ISBN 0130493465.