Chapter 3:
Tritone Substitution
What is a tritone? Known as the devil’s interval, its unsettling sound can be heard in countless suspense and horror movies. The resulting clash between these two notes was actually suppressed by the Church in the Middle Ages.
A tritone is the interval of three tones (three whole steps) between two notes. This interval can also be described as an augmented 4th or diminished 5th.
(In this chapter I’ll be using the word ‘tone’. Take this to mean an interval of a whole step.)
Play the notes F and B. Now turn it on its
head and play B and F:
They are both the same tritone!
The two notes of a tritone also form part of a diminished chord. You will recall that the interval between two notes of a diminished chord is a minor 3rd. A tritone is double this distance.
Tritones and dominant 7 chords
3 essential facts about tritones:
• A tritone and a dominant 7 chord are inseparable.
• This is because a tritone actually forms a part of the dominant 7.
• A tritone is the 3 and 7 of a dominant 7.
Play the chord Ab7.
The interval between C and Gb (3 and 7) is 3 tones.
This interval between 3 and 7 of a dominant 7 is therefore a tritone.
Fig 34 (below) shows you the Ab7 chord and its tritone.
Fig 35 contains six tritones. Each tritone ascends by a whole step.
In fact, fig 35 only contains three tritones. A2 is just an inversion of A1, and so on. Here they are again, now in pairs.
All these tritones belong to a dominant 7 chord. You will recall that the interval between 1 and 3 of a major triad is a major 3rd. (A dominant 7 is constructed with major and minor 3rds.)
Let’s start with A1.
* The bottom note is C.
* A major 3rd down from C is Ab.
* Therefore, the chord is Ab7.
Now take A2.
• The bottom note is F#.
• A major 3rd down from F# is D.
• Therefore, the chord is D7.
We now have two versions of the same tritone, each relating to its respective dominant 7.
Fig 37 shows each tritone alongside its related chord.
Do you see the connection between these two chords?
• They are a tritone apart.
• They share the same notes: 3 and 7.
• They are also connected by the same diminished scale!
This may not be immediately apparent, because some notes are enharmonic (same pitch but different name, e.g. Gb = F#).
Fig 40 (below) illustrates the connection more clearly by following each chord with its diminished scale. Remember, when playing a diminished scale over a dominant 7, the interval order is half step/whole step recurring.
Because of the close bond between these two chords, we can substitute one chord for the other. This is tritone substitution!
We can now state the following:
1. The chords involved are always dominant 7s.
2. The two related chords are a tritone apart.
3. They share one diminished scale.
4. They share the notes 3 and 7.
Practical use of tritone substitution
We know that a dominant 7 wants to resolve to its tonic: V wants to go to I.
I’ll now illustrate tritone substitution with a II – V – I sequence in G major.
The tritone substitution for D7 is Ab7. So now the sequence looks like this:
Notice how the chords descend by a half step. This makes life a lot easier when substituting the V chord. Rather than having to think up a tritone, we can just play the II chord, and then descend by a half step for the V chord.
More good news! Here is a 5-note comp over II – V – I in G major:
The D7 contains two altered notes:
Eb = b9
Bb = b13
Now see what happens when I play the same notes in the right hand but substitute the chord. (I’m now using 1 + 7 shell of Ab7 in the left hand).
The altered notes have gone! We now have 3, 5, 7 and 9 of Ab7.
C = 3
Eb = 5
Gb = 7
Bb = 9
In the next example, below, my phrase uses the diminished scale over the Ab7 tritone substitute chord.
Because tritone substitution is mostly used in a II – V – I situation, I could have just taught you to play the II chord and then move the V chord down by a half step to find the tritone substitute. Although this is a ‘quick fix’, it is only useful after grasping the theory.
One last word about tritone substitution: do not overuse it! Used in moderation, the effect is very appealing, but if you substitute every V chord, everything will start to sound the same. This also applies to vertical improvisation and rootless voicings. To use any of these techniques exclusively doesn’t make your playing any more sophisticated or advanced. They are only tools. A pentatonic or blues scale can sound just as effective as these more subtle sounds.
And speaking of blues, it’s time to take a 12-bar blues to the next level!
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