At the heart of Barry Harris's Theory of Evolution is the chromatic scale, shown on the top staff with enharmonic equivalents (i.e., different note names representing the same pitch, such as D sharp and E flat).Separate the solid and hollow noteheads, and you've got the two whole-tone scales (middle staff). Also derived from the chromatic scale are the three diminished 7th chords (bottom staff). Note that each includes two tritones: one from each whole-tone scale.

Each diminished 7th chord yields four dominant 7th chords. Just pick a note and lower it one-half step; this note then becomes the root of the dominant 7th chord. Shown below are the four dominant 7ths that relate to C#dim7 - Barry Harris would like you to keep the relationship between these four dominant 7ths in mind. Did the math already? That's right, three diminished 7th chords means twelve dominant 7ths in all.

From the diminished 7th chord you can also create major 6th chords (left) and minor 6th chords (right). These chords sound the same as their corresponding minor 7th and minor 7 flat 5 chords, respectively. For functional reasons that we'll cover in subsequent examples, think of them in their 6th forms.