When jazz musicians talk about “Rhythm Changes” they’re referring to songs that use the basic chord changes and structure of George Gershwin’s tune “I’ve Got Rhythm.” Next to the Blues, Rhythm Changes is one of the most commonly played song forms in Jazz music. There are literally hundreds (or more probably thousands!) of jazz songs that use these changes (or variations with substitutions).
Rhythm Changes are often played at very fast tempos, but thankfully can also be thought of quite simply; It’s an AABA form and the “A” section is basically key of Bb then a Bb7 to an Eb and back to Bb, while the “B” or “Bridge” is a cycle of Dominant 7th chords D7, G7, C7 to F7. There are many ways that turnarounds and substitutions can be used to embellish the changes, but at the heart of it, you’ll usually find these.
To master this form, a musician must have a plethora of melodies, licks, patterns a phrases at the ready. Here’s a nice phrase that fits over the A section. Notice how it outlines the turnarounds and targets the 3rds of the chords.
Practice this lick slowly at first. Then bring it up to tempo (about half-note = mm 150) using a metronome. It’s not necessary to pick every note. Try to keep the phrasing legato and horn-like!