What it is (from the standpoint of form)
Rhythm changes are the chord progression of George and Ira Gershwin's “I Got Rhythm” (musical Girl Crazy, 1930), which became, after the blues, the second most used form in jazz. Here we are interested in it as a form: the 32-bar AABA scheme, how it is arranged, how it is improvised over and how new themes (contrafacts) are built on the same structure. For the detailed harmony and substitutions see the entry “Rhythm changes (I Got Rhythm).”
The structure (in Bb, the traditional key)
| Section | Bars | Basic harmony |
|---|---|---|
| A1 | 1–8 | I–vi–ii–V turnarounds (B♭ Gm7 Cm7 F7) ×2, then I–I7–IV–iv (or #iv°) and I–V–I |
| A2 | 9–16 | same as A1, closes in the tonic |
| B (bridge) | 17–24 | III7 (D7) – VI7 (G7) – II7 (C7) – V7 (F7), two bars each: a chain of dominants by fifths |
| A3 | 25–32 | same as A2 |
Gershwin's original song has a 2-bar coda in the last A (34 bars in all); jazz musicians eliminate it and play a regular 32 bars.
The formal contrast
- The A sections are dense: one chord every two beats, fast harmonic rhythm, repeated turnarounds.
- The bridge is broader: one chord every two bars, “open” dominants that allow long phrases, altered tensions, changes of color.
This contrast is the heart of the form: the improviser “runs” in the A sections and “breathes” (or builds a long line) in the bridge.
Famous contrafacts
A contrafact is a new theme written over the chords of an existing piece. On rhythm changes:
- Charlie Parker / Dizzy Gillespie, “Anthropology”; Parker, “Moose the Mooche”;
- Sonny Rollins, “Oleo” (1954), with the bridge left to improvisation in the theme;
- Thelonious Monk, “Rhythm-a-ning”;
- Duke Ellington, “Cotton Tail” (1940);
- Lester Young, “Lester Leaps In”;
- the Flintstones theme (Hoyt Curtin) is also built on this form.
How to arrange rhythm changes for a small group
- Intro: often 4 or 8 bars of drums, or the I–vi–ii–V turnaround on piano.
- Head (theme): one or two complete choruses (AABA), with the theme in unison for sax and trumpet in the A sections; in the bridge the theme sometimes leaves room for a solo (as in “Oleo”).
- Solos: each soloist plays one or more 32-bar choruses; the harmonic rhythm is fast (typical bebop tempos: ♩ = 200–300).
- Trading fours / eights: soloists and drummer alternate every 4 or 8 bars.
- Head out and ending: often a tag (repetition of the last 2–4 bars) or a “stop” ending.
Signals of form: the musicians must feel where the bridge begins (bar 17): a drum fill or a change of groove (for example from swing to Latin in the A sections and swing in the bridge in some arrangements) helps everyone keep their place.
How to analyze a contrafact
- Verify that the form is a 32-bar AABA in B♭.
- Compare the melody with the chords: which notes fall on the strong beats of the I–vi–ii–V turnarounds?
- In the bridge identify the guide tones (F♯ over D7, B over G7, E over C7, A over F7).
- Note the substitutions suggested by the melody.
Exercise for a producer
Build a beat at 90 BPM (jazz-hop style) on the AABA structure of rhythm changes: A with the B♭ Gm7 Cm7 F7 turnaround sampled or played on Rhodes, bridge with D7 – G7 – C7 – F7 of two bars each, with a drum change (fuller kick, open hi-hat). Have someone rap or play a lead solo over the bridge: the AABA form automatically gives you a “verse–hook–bridge.”
Related
Rhythm changes (I Got Rhythm), The 32-bar American song form, Head–solos–head in jazz, Jazz standards and harmonic forms, Secondary dominants and extended dominants, Analyzing a jazz tune from a lead sheet.