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 common in the '40s and '50s, and are now ubiquitous.[7] First, "I Got Rhythm" was by then already a popular jazz standard. Second, by listening to the song and writing a new melody over its chord changes, thereby creating a composition of a type now known as a contrafact, a jazz musician could claim copyright to the new melody rather than acknowledge Gershwin's inspiration and pay royalties to Gershwin's estate. Third, using a stock, well-known progression for new melodies made it easier to perform a song at jam sessions, shows, and recordings because the bandleader could tell new musicians that the song uses rhythm changes and note any modifications and chord substitutions.
For contemporary musicians, mastery of the 12-bar blues and rhythm changes chord progressions are "critical elements for building a jazz repertoire".[8]
Chords
The rhythm changes is a 32-bar AABA form with each section consisting of eight bars, and four 8-bar sections.[9] In roman numeral shorthand, the original chords used in the A section are:
I vi
ii V
I vi
ii V
a 2-bar phrase, I−vi−ii−V (often modified to I–VI–ii–V), played twice,[10] followed by a 4-bar phrase
I I7
IV iv
I V
I
In a jazz band, these chord changes are usually played in the key of B♭[7] with various chord substitutions. Here is a typical form for the A section with various common substitutions, including bVII7 in place of the minor iv chord; the addition of a ii–V progression (Fm7–B♭7) that briefly tonicizes the IV chord, E♭; using iii in place of I in bar 7 (the end of the first A section); and using a ii-V-I in place of I-V-I in bars 15 and 16 (the end of the second A section):
Variant versions of changes are common due to the popularity of adding interest with chord substitutions, passing chords and changes of chord quality. Bebop players, for instance, would often superimpose series of ii–V (passing sequences of minor seventh and dominant seventh chords) or other substitutions for interest or in order to discourage less experienced musicians from "sitting in" on the bandstand. The opening I chord was often B♭6 in Gershwin's original, but beboppers changed it to B♭M7 or B♭7. For instance, the B section may appear as follows:[12]
An even more adventurous bebop-style substitution is to convert C7 | C7 | F7 | F7 to Gm7 | C7 | Cm7 | F7, and then to further develop this substitution by changing this to Am7 D7 | Gm7 C7 | Dm7 G7 | Cm7 F7.
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" and Juan Tizol's "Perdido" both use a different progression for the A section while using the rhythm changes bridge.[15] "Scrapple from the Apple" uses the chord changes of "Honeysuckle Rose" for the A section but replaces the B section with III7–VI7–II7–V7.
Other tunes use the A section of "Rhythm" but have a different bridge. Tadd Dameron's "Good Bait" uses the A section of the Rhythm changes but a different progression for the bridge.[16]
^ abcdYaffe, David (2006). Fascinating rhythm: reading jazz in American writing. Princeton, NJ: Princeton University Press. p. 17. ISBN978-0-691-12357-8. As well found in Olav Jullums composition 'Bedroom Leaves.'
^ abEllis, Herb; Holmes, Terry (1996). The Herb Ellis Jazz Guitar Method: Rhythm Shapes. pp. 4–5. ISBN9781576233412.
^Holbrook, Morris B. (2008). Playing the Changes on the Jazz Metaphor. Foundations and trends in marketing. Now Publishers Inc. p. 104. ISSN1555-0753.
^Rawlins, Robert; Bahha, Nor Eddine; Tagliarino, Barrett (2005). Jazzology: the encyclopedia of jazz theory for all musicians. Milwaukee, WI: Hal Leonard. p. 128. ISBN978-0-634-08678-6.Rawlins, Robert and Bahha, Nor Eddine (2005). Jazzology: The Encyclopedia of Jazz Theory for All Musicians, p. 128. ISBN9780634086786.
