Here’s how it works:
The purpose of a tritone substitution is to create an augmented and diminished pull towards a chord a 4th up, stronger than you would by just moving to it.
Most of the time this happens from chord I to chord IV, but you can find it in other places as well.
Step 1 – dom7
As you might know, in the blues we often turn the I chord into a dom7. This breaks away from the major scale’s I chord which has a natural extension of maj7.
In the key of A that means:
Amaj7: A – C# – E – G#
A7: A – C# – E – G
The D chord (chord IV) has the notes of: D – F# – A.
The G pull more towards the F# than the G# does as it is now only a semitone away.
Step 2 – The #5
To make the pull even stronger we can turn the A7 into a A7#5, we now have:
A7#5: A – C# – E# – G.
The E# (that’s an F) want to move towards an F# stronger than the original E, again, because it’s now only a semitone away.
Step 3 – The b9
To go even further we can add a b9 to the A chord, you now have this:
A7b9: A – C# – E – G – Bb.
The Bb wants to go to the A as it’s only a semitone away.
You could also try #9 and b5, all these are augmented and diminished notes and create a similar sound.
Step 4 – Tritione substitution
Instead of building chords like A7#5, A7b9, A7b5, A7#9 or even A7b5b9 etc you can tritone substitute.
A tritone is a #4 or a b5 (same thing). From an A this is the note D#.
If we take the note D# and play a dom7 chord, ie D#7 we have tritone substituted.
The notes of an D#7 are:
D#7: D# – G – A# – C#
In relation to A these are:
A Tritone substitution: D# (b5) G (b7) A# (b9) C# (3).
We get the same pulling effect using this method as we would extending our A chord to A7#5, A7b9 or any of the other slightly bonkers sounding combinations.
Tritone substitute when soloing
When soloing, if you want the “outside sound” the augmented/diminished notes have you could instead of learning those arpeggios just tritone substitute.
Try it by playing a lick over A, using Mixolydian or major pentatonic, then just before you move to the D chord, add the arpeggio of D#7.
Tritone substitution is a great tool to use when you want to achieve an “outside sound”.
It highlights notes like #5 and b9
You can do this with chords but also when soloing.
For more about music theory, learning arpeggios and scales, check out the Guitar Conspiracy.