Barry said: “I don’t believe in modes”

It’s important to emphasize that Barry did not believe in using modes to create harmony. On the contrary, he focused almost exclusively on the three most important harmonic functions in music: Tonic, Dominant and Subdominant. For this reason, all the exercises on the sixth diminished scale described in Lesson 3 should be practiced on both the Tonic and the Subdominant.

Dominants require a special treatment.

Here is the rule:

For altered dominant chords, play a minor sixth diminished on the b2 of the dominant chord.
Moving up and down this scale highlights b9, #9, b5, #5, and 13, in addition to the root, 3rd and b7th (8 notes in total). Note: Barry did not use a ‘seventh diminished’ scale.

Here is an example of using the minor sixth diminished scale on a G7 dominant chord

Although Barry never used the word ‘altered’, we will name this chord G7alt since it contains all the notes of the altered scale (plus the extra E).

So, for G7 alt, play Ab minor 6 over G7 (as in example above).

As we will discuss in more detail later, Ab minor 6 is the ‘important minor’ of a Db7, which is the tritone of G7. Barry called it the tritone’s minor.

The three harmonic functions, I, IV and V,  will cover all the harmonies found in the typical bebop repertoire (which we will discuss in Lesson 6).

For example:

D minor 7 b5 = F minor 6 (in 1st inversion)
G7 = Ab minor 6 (with G in the bass)

With this equivalence in mind, a simple progression Dm7b5 -> G7alt becomes Fm6 -> Abm6

Here is an example of Barry playing a Bb-6 -> Db-6 using the minor sixth diminished scale. The example is on Star Eyes:

Also: D minor 7 = F major 6 (in 1st inversion)
With this equivalence in mind, the progression Dm7 -> G13b9 -> C6 becomes F6 -> G7b9 -> C6:

After a D-7 (or F6), Barry often played a G13b9b5. This could be described as a diminished-diminished, or double diminished, although Barry never used these terms. The reason he liked this is because the voicing for D-7 became G13 b9b5 by moving every note down a half step:

This is also a great comping pair!

The advantage to reducing all the basic chords (Maj7, 7, m7, m7b5) to major or minor 6 chords is that you can now apply the sixth diminished scale to each of these chords. Barry showed his students hundreds of exercises just on these two scales, and so the task of harmonizing any standard is reduced to learning movements on just two types of chord, rather than 4 and more.