Syncopation (or syncopated rhythm) in music involves the stressing of a normally unstressed beat or subdivision of a beat. This can be accomplished either positively, by emphasising the relevant note, or negatively, by omitting a note expected to be stressed. Syncopation thus gains its effect only in musical traditions that make use of strong, regular rhythms, as is the case in most Western music.
Syncopation can be heard as an occasional effect in most Western musical styles, including Western classical music; it is, however, an essential part of styles such as ragtime, jazz and reggae, all influenced by African music. In modern popular music, of which blues and jazz are important roots, syncopation is most often heard as a "back beat".
In the Baroque period, larger ensembles and the development of standardised forms of musical notation started a trend towards composers relying on styles of composition which strongly emphasised the first beat of a measure. This constantly heavily accented first beat, while useful and normal sounding, could also be an artistic restraint. The classical masters popularised the use of stressing the off-beat as a way of energising and breaking up the predictability of the strong first beat.
A more subtle form of manipulating the rhythm or the beat is a technique called "hemiola", or "cross rhythm", in which a short passage shifts the accent within the measure to suggest a time change from triple time (a three-beat measure) to double time (a two-beat measure) and vice versa without the composer actually writing in a time change.
European and American folk music have had an important influence on the use of syncopation in Western music in the twentieth century. Composers such as Aaron Copland, Ralph Vaughan Williams, and Percy Granger all wrote or adapted familiar-sounding tunes and employed strong off-beat syncopation. Meanwhile composers like Igor Stravinsky and Bela Bartok adopted a primitive notion of rhythm in which uneven patterns of off-beats were accented.
In later twentieth-century music, composers have experimented with unusual time signatures and free rhythm to further de-emphasise the idea of the standard beat.