Why do jazz musicians swing their eighth notes?

In jazz, the function of durational inequality at the eighth-note level is the production of anacrusis on the offbeats, thereby generating the sense of forward propulsion and drive thought to typify the rhythmic quality known as “swing.” The common use of relatively “straight” eighth notes by improvising soloists helps to sustain forward momentum, whereas the less even, triplet-like “swing” eighth notes used more frequently by drummers facilitate the perception of a quarter-note beat. Varying the Beat-Upbeat Ratio (BUR)—i.e., moving between straight and swing eighth notes—enables jazz musicians to manipulate the flow of motional energy across a phrase in systematic ways in conjunction with other melodic processes. Analyses of melodic phrases by John Coltrane, Lee Konitz, and Sonny Clark reveal the centrality of such aspects of microrhythmic expression to the affective power of jazz improvisation.

https://doi.org/10.1525/mts.2011.33.1.3

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