Introduction
One of the most common Chord Substitutions in Jazz is the Tritone Substitution. This is a way of substituting V7 chords. So a G7 would become a D♭7 (the root note is a tritone away). They work because the Guide Tones (3rd & 7th) are the same in both chords.
- G7 = G B D F
- D♭7 = D♭ F A♭ B
Tritone Substitution and Scales
Ordinarily, you would just use the C Major Scale (or equivalent modes) to improvise over a II-V-I in C, as shown in Chord Progression #1 below.
| ii | V7 | I | |
|---|---|---|---|
| Chord Progression #1 | Dm7 | G7 | CMaj7 |
| Scales | D Dorian | G Mixolydian | C Ionian |
| Key | C Major | ||
| Chord Progression #2 | Dm7 | D♭7 | CMaj7 |
| Scales | D Dorian | D♭ Mixloydian | C Ionian |
| Key | C Major | G♭ Major | C Major |
| Chord Progression #3 | Dm7 | G7 - D♭7 | CMaj7 |
| Scales | D Dorian | ? | C Ionian |
But what if you were to tritone substitute the G7 to D♭7, like in Chord Progression #2 above? Then you cannot (generally) use the G Mixolydian mode over the D♭7 because it contains the note C (which is a natural 7th from the point of view of the D♭7). You could, however, play the D♭ Mixolydian mode (G♭ Major).
- G Mixolydian (C Major) = G A B C D E F
- D♭ Mixolydian (G♭ Major) = D♭ E♭ F G♭ A♭ B♭ C♭
But what if the chord progression was 1/2 a bar of G7 and 1/2 a bar of D♭7, like in Chord Progression #3 above? Or what if someone was ‘comping and you didn’t know which chord they were going to play?
- D♭ Mixolydian doesn’t quite fit over the G7 chord; and
- G Mixolydian doesn’t quite fit over the D♭7 chord
And changing scales for 1/2 a bar, especially at a quick tempo, can be challenging and unnecessarily complex.
Wouldn’t it be great if there was a scale that fit over both G7 AND D♭7? Well, there is. You have a number of options when improvising over G7 and/or D♭7:
- Play G Mixolydian but avoid the C
- C is an avoid note for G7 and D♭7 – so just omit it
- Scales that fit over G7 and D♭7
- Wholetone Scale = G A B D♭ E♭ F
- Dominant Diminished Scale = G A♭ B♭ B D♭ D E F
- A♭ melodic minor = A♭ B♭ C♭ D♭ E♭ F G
- AKA G Altered Scale
- AKA D♭ Lydian Dominant
- D melodic minor = D E F G A B C#
- AKA D♭ Altered Scale
- AKA G Lydian Dominant
So now it doesn’t matter whether the accompaniment plays a G7 or a D♭7, because the above scales work well over both chords.
Miscellaneous Dominant Facts
A few more interest points about tritone substitution:
- A V7♭5 Chord is its own tritone substitution
- G7♭5 = G B D♭ F = D♭7♭5 = D♭ F G B
- Rootless Voicings of Dominant Chords include the: 3rd, 7th, 9th & 13th (we will discuss rootless voicings a future lesson)
- Rootless G7 = F A B E
- Rootless G7#9♭13 = Rootless D♭7 = F A# B E♭
- You will often see Tritone Substituted II-V‘s
- When improvising, use the relevant Dorian scale for m7 chords
- The possible combinations and permutations are listed below
| Dm7 | G7 | CMaj7 |
| A♭m7 | D♭7 | CMaj7 |
| Dm7 | D♭7 | CMaj7 |
| A♭m7 | G7 | CMaj7 |
