Rhythm is the variation of the duration of sounds over time. When governed by rule, it is called metre. It is inherent in any time-dependent medium, but it is most associated with music, dance, and most poetry. The study of rhythm, stress, and pitch in speech is called prosody; it is a topic in linguistics. All instrumentalists deal with rhythm, but it is often considered the primary domain of drummers and percussionists.
In Western music, rhythms are usually arranged with respect to a time signature, partially signifying a meter. The speed of the underlying pulse, called the beat, is the tempo. The length of the meter, or metric unit (usually corresponding with measure length), is divided almost exclusively into either two or three beats, being called duple meter and triple meter, respectively. If each beat is further divided by two it is simple meter, if by three compound meter.
Some genres of music make different use of rhythm than others. Most western music is based on divisive rhythm, while non-western music uses more additive rhythm. African music makes heavy use of polyrhythms, and Indian music uses complex cycles such as 7 and 13, while Balinese music often uses complex interlocking rhythms. By comparison, a lot of western classical music is fairly rhythmically simple; it stays in a simple meter such as 4/4 or 3/4 and makes little use of syncopation. In the 20th century, composers like Igor Stravinsky, Philip Glass, and Steve Reich wrote more rhythmically complex music using odd meters, and techniques such as phasing and additive rhythm. At the same time, modernists such as Olivier Messiaen and his pupils used increased complexity to disrupt the sense of a regular beat, leading eventually to the widespread use of irrational rhythms in New Complexity. LaMonte Young also wrote music in which the sense of a regular beat is absent because the music consists only of long sustained tones (drone).
A rhythm section generally consists of percussion instruments, and possibly chordal instruments (e.g., guitar, banjo) and keyboard instruments, such as piano (which, by the way, may be classified in any of these three types of instruments).
"Rhythm," wrote Tom Robbins in 'Another Roadside Attraction', "is everything pertaining to the duration of energy."
