The essential tones (3 and 7) of the major 7th chord are a perfect 5th apart.
The essential chord tone combinations for each major chord are therefore unique.

The essential tones (b3 and b7) of the minor 7th chord are also a perfect 5th apart.
The essential chord tone combinations for each minor chord are therefore also unique.

(But each essential chord tone combination serves both one minor chord and one major chord.)

However the essential tones of the dominant 7th chord are a tritone (3 whole tones) apart. This interval is symmetric : when inverted the interval remains the same.

This means that the 3 and b7 of one dominant chord are the same as the b7 and 3 of another dominant chord.
In other words there are two chords with the same dominant chord quality that share the same two essential tones.
This relationship between the two chords is so strong that they can be interchanged, substituted, for one another.
This is called Tritone Substitution.

• E is the 3rd of the C7 chord and is the 7th of the Gb7 chord
• Bb is the 7th of C7 and the 3rd of Gb7

Note that the two dominant chords which can substitute each other are exactly opposite each other on the Circle of Fifths

In practice you can use the Tritone Substitution principle on any dominant 7th chord. Simply replace it by the dominant 7th chord with its root tone a tritone (6 semitones) away.

• C7 can become Gb7, or vice versa
• G7 can become Db7, or vice versa
• Eb7 can become A7, or vice versa

  and so on.

Whether the chord substitution in a given musical context is appropriate or not should be judged by your own musical ear.

JT 19.5 - Chromatic Chord Progressions

One can apply Tritone substitutions to the Circle of Fifths, substituting alternate chords on a circle of dominant 7th chords.

1. This manipulation transforms the Circle of Fifths into a Circle of Semitones, a Chromatic Circle.

   Chord progressions that move stepwise down in semitones are therefore tritone variations of chord progressions that follow the Circle of Fifths.
   This is why chromatic chord movements always sound good.

   Two examples :

   1. In
   Satin Doll (bars 7 and 8) : C7 - B7 - Bb7 - A7 

2. In Autumn Leaves (6 bars before the end) : Em7 - Eb7 - Dm7- Db7 

JT 19.6 : The IIm7 - bII7 - Imaj7 Progressions

An obvious target for Tritone Substitution is of course the IIm7 - V7 - Imaj7 chord progression.

Substituting the V7 chord (G7 above) for the bII7 chord (Db7) produces the progression :

Audio 11

This substitution also requires a new improvisation scale for the substitute chord, the bII Mixolydian mode.

The new scale gives a nice contemporary sound to the improvisation (but also requires more practice to improve your improvisation skills).

Keyboard players note that in a IIm7 - bII7 - Imaj7 chord progression all three chords can be played in root position (as above) or in the same inversion.

Summing up : an understanding of chord substitutions has three major benefits.

1. It helps you to understand and analyse the chord progression of any song much better.
2. It provides keyboard players and guitarists with a much wider selection of chords to use for accompaniment.
3. It enables you to change boring chord progressions by adding some spice.
   (When playing in a band always discuss and agree on such changes with all band members).