There is more than one way to play a blues; not the musical genre, but the typically 12-measure chord sequence that every jazz musician has played somewhere along the way. Those famous changes, in the key of C: four measures of C7, two of F7, two of C7, two of G7, and then two more of C7. These changes form the most important chord sequence in jazz. But it's not always done the same way. In fact, it's played a little differently almost every time.
Let's look more closely. Here's the "basic" chord sequence, written all in sevenths:
C7 | C7 | C7 | C7 | F7 | F7 | C7 | C7 | G7 | G7 | C7 | C7
Let's take a look at the first four measures of a blues, which in their simple form go like this:
C7 | C7 | C7 | C7
That's a lot of the same thing. So jazz musicians often replace the second measure's C7 with an F7, so the first measure is C7 and the second is F7, but the third and fourth are C7. Elementary.
Now, we look at the fifth and sixth measures and there's one change that can be made:
F7 | F7 is changed to F7 | F#dim7
This is an interesting chord change, because on the surface the two chords are quite different: for a start, wouldn't F in one chord and F# in the next chord clash?
That's where the magic of it comes in. F7 contains the notes F, A, C, and Eb. As it turns out, the notes in the F#dim7 chord are very similar - only the root note is different, so the notes in the chord are F#, A, C, and Eb. So if you played both chords at once (root and all, so you can hear the transition), you'd see that going from F7 to F#dim7 just requires moving the root note up from an F to F#.
Now come the next four measures, which mark the climax of a blues:
C7 | C7 | G7 | G7
Since this isn't much of a climax, we add chords here one of two ways, either:
C7 | C7 | G7 | F7 or C7 | A7 | Dmin7 | G7
You can use either set of chord changes, depending on the tune you're playing. Now comes the "turnaround" section of a blues, the final two measures, which we originally in this article wrote as just:
C7 | C7
We instead can make this into a few different things: we can change the last measure to G7, or even do a turnaround like the "climax":
C7 A7 | Dmin7 G7
So that finishes the "basics" and gives us the following, more complex chord sequence:
C7 | F7 | C7 | C7 | F7 | F#dim7 | C7 | A7 | Dmin7 | G7 | C7 A7 | Dmin7 G7
But there's much, much more you can do with a blues to make it more interesting. One variation of the blues is often called the "16-bar blues"; this is how Jamey Aebersold describes his "Watermelon Man" play-a-long in Maiden Voyage. The chord sequence he uses in that sheet music for the tune includes four measures of F7, two of Bb7, two more of F7, and then six measures going back and forth between C7 and Bb7 before returning to F7. Basically, "Watermelon Man" keeps the climax going for an extra four measures to make it longer. The interesting thing about making a 16-bar tune is this: if you play it twice, the total length is 32 measures, the typical length of jazz standards (see https://en.wikipedia.org/wiki/Thirty-two-bar_form).
Another choice available is this (written four measures per line to make reading easier):
C7 | C7 | C7 | C7
F7 | F7 | F7 | F7
C7 | C7 | C7 | C7
G7 | F7 | C7 | C7
Both of these examples - "Watermelon Man" and the example directly above" - make a certain section of the blues longer so that the whole thing adds to 16 measures rather than 12. But there's a whole different take on it all, and that's the 3/4 time version (8 measures written per line):
C7 | C7 | C7 | C7 | C7 | C7 | C7 | C7
F7 | F7 | F7 | F7 | C7 | C7 | C7 | C7
G7 | G7 | F7 | F7 | C7 | C7 | C7 | C7
The 3/4 version sends us to "West Coast Blues", a blues written in 3/4 time. The Real Book sheet music is in the key of Bb, but we'll explain the sequence as if it was in C so it is easy to understand.
The Real Book uses these chords (transposed to C) for the first eight measures of the tune: two measures of C7, then two of Bb7, two more of C7, and then a measure of C#min7 and then one of F#7; this makes the transition to F7. This part of the whole chord sequence for "West Coast Blues" is quite original, but also makes a lot of musical sense.
The first transition of "West Coast Blues" from C7 to Bb7 is also done in Benny Golson's "Killer Joe" and, as mentioned above, "Watermelon Man". It's a good tension-builder, but what's interesting about its use in "West Coast Blues" and "Killer Joe" is that it's used right at the beginning of the tune. This gives West Coast Blues a unique sound as soon as it begins - but not in a way that's so noticeable that it distracts from everything else, unless you listen very closely.
Another interesting collection of chords is, after the second set of two measures of C7, the chords C#min7 and F#7. See the musical sense in going from the two measures of C7 to a C#min7 chord? Remember, earlier in this article, when we mentioned why you can go from an F7 to an F#dim7 in a blues? The same is happening here, but instead earlier in the chord sequence.
Then we quickly go up from a C#min7 chord to an F#7, which is of course a common chord change. And then the genius of getting to the desired destination, F7, through all this - just go down from an F#7 to an F7 because an F#7 can serve as an equivalent for C7. So we successfully make the transition from C7 to F7 by going to a C#min7 and then a F#7 - all the while, keeping standard and logical chord changes in place. Genius, either on the part of the Real Book or Wes Montgomery. It would be great to see chord changes like these being used more in the blues today.