What Is A Tritone Substitution?
First of all what’s a substitution? A substitution is a chord we substitute for another chord. Both chords will have some common notes and share a similar function.
When we substitute one chord for another we’re basically looking to keep the same feel and direction but the substitution is going to provide a different colour.
Here we have a ii-V-I progression in C Major:
Dm7 – G – Cmaj7
If we play a D7 in place of the Dm7 we get the following progression:
D7 – G7 – Cmaj7
The D7 is a ‘substitution’. In this case, it’s what we call a Secondary Dominant substitution.
Next we have the term tritone. What’s a tritone?
Well, think of the name Tri-Tone. That’s three tones or three whole steps.
If we measure three whole steps from C we get the following:
C – Up one tone – D – Up one tone – E – Up one tone – F#
So C to F# is a tritone. We can also see this as Gb and the flattened 5th is usually more appropriate than the augmented 5th. So in measuring a tritone from the note C, we have Gb. We can measure a tritone from any note eg. G to Db.
The Dominant 7 Factor!
Tritone Substitutions are all about the dominant 7 chord. We simply substitute a dominant 7 chord with another dominant 7 chord a tritone away.
So, for a C7 chord the tritone substitution would be Gb7.
Tritone Substitutions are very common in ii-V-I progressions as a substitution for chord V.
Below we see the ii-V-I progression and the tritone substitution in C Major:
- Dm7 – G7 – Cmaj
- Dm7 – Db7 – Cmaj7
As you can see, the tritone substitution provides a smooth chromatic movement in the bass line. We simply move down from D to Db to C.
Why It Works
The tritone substitution works because of another important tritone interval.
Let’s look at the notes of the C7 chord:
C E G Bb
The two most important colour notes for a dominant 7 chord are the 3rd and 7th. In this case, E and Bb. These notes are a tritone apart.
Next let’s look at the notes of the tritone substitution: Gb7
Gb Bb Db Fb
The Fb in this chord could also be seen as E. If we look at the colour tones again, the third and the seventh, we have the notes Bb and E. Exactly the same tritone as we had in the C7, but inverted.
This means that both C7 and Gb7 share the same colour tones of E and Bb. This important tritone interval is the same in both chords and the reason they work as interchangeable chords so well.