From The Art and Popular Culture Encyclopedia
In jazz and jazz harmony, "rhythm changes" refers to the chord progression occurring in George Gershwin's song "I Got Rhythm". This pattern, which forms the basis of countless (usually uptempo) jazz compositions, was popular with swing-era musicians: It is found in "Shoeshine Boy" (Lester Young's 1936 breakout recording with Count Basie) and "Cotton Tail" written by Duke Ellington in 1940, as well as Charlie Christian's "Seven Come Eleven", "Charlie Parker's 'Salt Peanuts'," and Thelonious Monk's "Rhythm-a-Ning", for instance. The earliest known use of rhythm changes was by Sidney Bechet in his September 15, 1932 recording of "Shag" with his "New Orleans Feetwarmers" group.
This progression's endurance in popularity is largely due to its extensive use by early bebop musicians. The chord changes began to be used in the 1930s, became extremely common in the '40s and '50s, and are now ubiquitous. First, "I Got Rhythm" was by then already a popular jazz standard, and by listening to the song and writing a new melody over its chord changes (a type of composition known as a contrafact), a jazz musician could claim copyright to the new melody rather than acknowledge Gershwin's inspiration and pay royalties to his estate.
In popular music, "rhythm changes" refers to the first four-chord section of the full progression that the term denotes in the jazz context. This "sub-progression" forms the entire harmonic structure of an enormous number of popular hits released during the 1950s and 1960s (as well as of a significant number since then), in part because Gershwin's song is not the only source of this portion of the progression: Because the chord changes in question form part of a circle progression, they have been readily amenable to independent discovery by other artists both before and after Gershwin, with the effect that less creative musicians have been able to "borrow" the progression from songs other than "I Got Rhythm" whether or not those songs themselves borrowed from Gershwin.
David Yaffe argues that the use by African-American musicians and groups of a chord structure written by a Jewish musician is a highly visible example of the collaboration between the groups that took place in song more than in written literature due to racial segregation.