The songs we are requested to improvise over are not always twelve bar blues with a simple I7 IV7 V7 harmonic structure… the harmonies can sometimes be very challenging.

A typical example in jazz, gospel, ragtime, soul and even classical, is when you have a sequence of dominant 7th chords that progress by intervals of a perfect 5th. I call them “cascading dominant 7th chords”.

Let’s take the song “Georgia On My Mind”. It consists of four sections: A A’ B A. Let’s have a look at the A section:

|G |B7 |Em G7/D |C C#°7 |
|G E7 |A7 D7 |F7 E7 |A7 D7 |

The song is in G major and starts with the I chord, immediately followed by B7 which is a secondary dominant chord: it is the V7 of vi, i.e. Em.

Then we have another secondary dominant (G7) resolving to C (the IV chord).

The C#°7 chord (#IV°7) is a chromatic passing chord that resolves to I (G). You can also view it as a #I°7 chord, because of the symmetry of dim7 chords.

Next comes a series of dominant 7th chords that doesn’t resolve: E7 A7 D7. You can analyze them as
V/V/V -> V/V -> V

And then another series: F7 E7 A7 D7 -> G
F7 is equivalent to a B7: it is a tritone substitution. So this bit can be analyzed as
bII/V/V/V -> V/V/V -> V/V -> V -> I.

The question is: how do we treat those cascading V7 chords when we improvise?
Here are three possibilities:

1. Target notes
One possibility is to target those notes that are clearly not in the key of the song. They are the notes that make the chords stand out, and so it makes sense to highlight them.
For example on E7, that would be the G#, on A7 that would be the C#, etc.
As a general rule, you can always target the chordal 3rds and b7ths.

2. Blues scale of the target chord
The sequence E7 -> A7 -> D7 seems to resolve on G (although it doesn’t actually resolve: the resolution is implied). Therefore, it is possible to use the G blues scale over that sequence:
G B C (Db) D F. This scale is the G minor pentatonic with an added b5.
Doing this will create tensions, but it is harmonically consistent and will sound great if done well.

3. One blues scale per chord
Another approach is to use the blues scale corresponding to each passing V7 chord.
So, for the sequence B7 E7 A7 D7, you could use:
– B blues (B D E F F# A) over F7
– E blues (E G A Bb B D) over E7
– A blues (A C D Eb E G) over A7
– D blues (D F G Ab A C) over D7, returning to G
In all these blues scales, the b3, 3 and 5 work particularly well.
As above, you could also use the G blues scale over all these chords.

I hope this will help some of you tame those cascading V7 chords!