In jazz lingo, "rhythm changes" refers to the chord progression from the George Gershwin tune "I Got Rhythm." Bebop pioneers like Charlie Parker, Dizzy Gillespie, Thelonious Monk, and Bud Powell wrote numerous new melodies over this progression and embellished the original chords as well.
This progression has, over the years, become an essential progression (along with the blues progression) for students of jazz to learn. Making all the changes in this tune can be quite a challenge, especially since it's usually played at very fast tempos. As with approaching anything new on guitar, the key is to break the progression down into digestible chunks and work on it at tempos that are comfortable at first. The speed will come with time and familiarity.
What follows are some suggestions for approaching basic soloing over rhythm changes and some ideas for extending the basic approach as well.
Many variations on the rhythm changes sequence exist, but it's safe to say that this version of the A section falls somewhere in the ballpark of common practice:
| Bbmaj7 Bdim | Cm7 Dbdim | Dm7 G7b9 | Cm7 F7 |
| Fm7 Bb7 | Ebmaj7 Edim | Dm7 G7b9 | Cm7 F7 ||
For the second A (and the last A on the head out), replace the last two bars with:
| Cm7 F7 | Bbmaj7 ||
To start with, I hear a lot of people refer to "diminished passing chords" in classical as well as jazz theory. To me, this has always kind of seemed like only half the story. In the first two bars, the B dim and Db dim could be thought of as simply passing chords to get up to the next chord, but I've always found it more useful to think of diminished chords as having (usually, but not in every case, of course) one of two possible functions:
1. substitute for a V chord that resolves to the next chord. For example, the Bdim chord resolves to Cm, so to me it's just a sub for G7b9 (the notes in Bdim are B,D,F,Ab, which make up a G7b9 chord without the root.) Here the default scale would be C harmonic minor (C,D,Eb,F,G,Ab,B).
2. part of a I diminished to I major progression. This is what's going on in the second measure (Dbdim to Dm7). Bbdim (Bb,Db,E,G) is the same as Dbdim (Db,E,G,Bb), and Dm7 (D,F,A,C) is just Bbmaj7(9) without the root.
So, as far as scales/key centers over the first two bars (and into bar 3), you'd get this:
| Bb C harm | C harm Bb dim | Bb...
Notice that I put C harm as the scale over Cm7 even though this creates a clash between the B in the scale and the Bb in the chord. This chord is functioning both as a temporary I chord and as a ii chord in Bb, and especially at a faster tempo, the clash isn't going to be a problem. Even at a slower tempo, if you treat the B as a passing tone and don't sit on it really hard, it will still work fine over a Cm7.
You can try emphasizing the differences between a few of the notes from chord to chord (particularly the differences between the diatonic home key of Bb and the other scales). For example, the difference between the Bb scale and the C harm minor scale is that the Bb scale has an A and a Bb, and the C harm minor has an Ab and a B natural. If you emphasize the new notes, you'll help bring out the sound of the underlying harmony. Between Bb dim and Bb major, there are several differences:
Db, E, and F# in Bb dim
D natural, Eb, and F natural in Bb major
An alternate idea would be to think of the Db dim (Db,E,G,Bb) as an A7(b9) (A,C#,E,G,Bb) with no root (here it would be the V chord of D minor). This way, you'd get a progression a whole step above the G7(b9) to Cm7, and you could make use of this by playing one line over the first two chords, and then move it up a whole step and play the same line over the next two.
After this, there's a "straight-ahead" turnaround in Bb. Next, you have a ii V into Eb major, which is pretty self explanatory. The E diminished chord progresses to Dm7 (some versions will have this written as Bbmaj7/F). Here, we have the same I diminished to I major progression in disguise; E dim (E,G,Bb,Db) is the same as Bb dim (Bb,Db,E,G), and Dm7 is a sub for Bb major. Then there's another turnaround in Bb. Anyway, this is just sort of a thumbnail sketch of the "regular" way to play over the A.
A few ideas for substitutions:
The tritone sub as in "Lady Bird":
| Bbmaj7 Db7 | Gbmaj7 B7 | ...
Here, you're going from the I chord immediately into the key of Gb (for four beats) and then to a tritone sub for F7 (usually B Lydian dominant, also known as Gb melodic minor, which is only one note different from Gb major).
The descending minor chord:
| Dm7 Dbm7 | Cm7 Dbdim | Dm7 Dbm7 | Cm7 F7 | ...
Here, you'd typically play some sort of pattern, like maybe an arpeggio, and take it chromatically down over the three minor seventh chords.
The cycle of fifths:
| F#7 B7 | E7 A7 | D7 G7 | C7 F7 | Fm7 Bb7...
Most people using this idea will play a fairly simple repeating cycle such as 1,2,3,5 over each dominant chord. Sonny Stitt and Bud Powell both used this device often, and Bill Evans used this progression exclusively for the solo section on the original recording of his rhythm changes tune "Five."
Another possible variation of this progression:
| C7 F7 | Bb7 Eb7 | Ab7 Db7 | Gb7 B7 | Fm7 Bb7...
(this one's just a tritone sub of the first one)
| F#7 F7 | E7 Eb7 | D7 Db7 | C7 B7 | Fm7 Bb7...
You can simplify the harmony, particularly when the tempo is pretty fast:
| Bb7 | | | | Fm7 Bb7 | Ebmaj7 Edim...
Over the Bb7, you could use any of the harmonies associated with a dominant chord such as diminished dom or altered dom. You could think of the first four bars as the tritone sub, E7 as well.
| F7 (B7) | | | | Fm7 Bb7 | Ebmaj7 Edim...
Here, you're thinking of the first four bars as a big V chord (F7or it's tritone sub, B7) finally resolving to the Bb chord in bar 5.
You could take it a step further and completely eliminate chord movement:
| Bb7| | | | | | | |
This could be a good tension-builder for even a whole chorus, especially if the rhythm section is hip to what you're doing and pedals on the Bb. I've heard people use Bb diminished in this way as well. You could also do the same thing over F7.
As for the bridge, you can try using the tritone sub idea here:
| D7 | | Db7 | |
| C7 | | B7 | |
With all the extra elbow room over the chords, it might sound hokey to repeat the same two bar phrase verbatim down chromatically, but a slight paraphrase each time can be effective. Of course, you can do the opposite as well:
| Ab7 | | G7 | |
| F#7 | | F7 | |
or combine the two:
| D7 | Ab7 | G7 | Db7 |
| C7 | Gb7 | F7 | B7 ||
Typically, over all of these dominant chords, you'd play the standard bebop II V licks with chromatic passing tones galore, though you can certainly make use of the altered, Lydian dominant (for the tritone subs), and diminished dominant scales here, as well.
One last note:
When the tempo heads for the stratosphere, it's really important to develop the ability to see the big picture as opposed to thinking of each chord or couple of chords. In order to be able to make a coherent musical statement at really burning tempos, you should probably have the tools ingrained enough that you're not really thinking about them and can be free to concentrate on the larger shape of your lines in the context of four bars at a time, eight bars at a time, or even a whole chorus at a time.
A more complete exploration of rhythm changes is available in my two-part class Rhythm Changes: A Modern Jazz Guitar Approach part 1 and Rhythm Changes: A Modern Jazz Guitar Approach part 2 from Mike's Master Classes.