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Tritone Substitution

Tritone substitution is another form of chord substitution. The name “Tritone” means three tones (or three whole steps) which is a diminished 5th (or augmented 4th) interval. This diminished 5th interval shows up in dominant 7 chords and in diminished chords. However, for tritone substitution only dominant 7 chords are used.

The basic idea is to take the dominant 7 chord (for example G7 in the key of C major) and replace it with another dominant 7 chord that has the same diminished 5th interval (tritone) but “flipped”. For example:

G7: G B D F

B - F is the tritone in G7, so the chord we are looking for will have F – B as the tritone. That chord just happens to be a tritone (3 whole steps) away from the G7 chord, which is Db7.

Db7: Db F A Cb(B)

In this case you are barrowing a chord from outside of the key of C major, but since the Db7 has the same tritone interval, it has a similar function to the G7... That is, it will pull towards C (the tonic).