Their later popularity is largely due to their extensive use by early bebop musicians. "I Got Rhythm" was already a popular jazz standard, and by writing a new song over its chord changes (a type of composition known as a contrafact), the tune could be copyrighted to the artist instead of requiring that royalties be paid to the Gershwin estate.
In popular music rhythm changes refers to just the first four chords of the jazz progression. These form the total harmonic basis of an enormous number of popular hits that came out during the 1950s, 1960s, and beyond.
"Rhythm changes" are a thirty-two-bar form. In Roman numeral shorthand, the actual chords used in the "A" section are | I vi | ii V | (tonic-submediant-supertonic-dominant) played twice, then | I I7 | IV #iv°7 | I V | I |
(or | I I7 | IV iv |, which is what Gershwin originally wrote)In C major, for example, these chords would be | C Am7 | Dm G | C Am7 | Dm G | | C C7 | F F#° | C G | C | (or | C C7 | F Fm |) The "bridge" consists of a series of dominant sevenths that follow the circle of fifths, sustained for two bars each and thus conveying the sense of a shifting key center: | III7 | ÷ | VI7 | ÷ | | II7 | ÷ | V7 | ÷ | In our example, we begin with an E7, followed by an A7, then D7 and finally G7, bringing us back to the original key for a final reprise of the A section. A two-bar "tag" at the end of the Gershwin tune is generally omitted. While rhythm changes can be played in any key, they are most commonly played in concert B-flat and sometimes E-flat.
Variant versions of the A section changes are legion: often the beboppers, for instance, would superimpose series of "two-fives" (passing sequences of minor-7th and dominant-7th chords) on the A section in order to make things interesting for themselves (and in order to discourage lesser musicians from sitting in on the bandstand).
The component A and B sections of rhythm changes were also sometimes used for other tunes: for instance, Charlie Parker's "Scrapple from the Apple" uses the chord changes of "Honeysuckle Rose" for the A section, but replaces the B section with "Rhythm"'s | III7 | VI7 | II7 | V7 | bridge. Other tunes, such as Sonny Stitt's "The Eternal Triangle", or "the Muppet Show Theme", use the A section of "Rhythm" but have a different bridge. Often in rhythm changes tunes, the B section is left free for improvisation even during the head (e.g. in Sonny Rollins' "Oleo").
The following is a partial list of songs based on Rhythm Changes.