Tritone substitution – one of the many chord substitutions used in Jazz
Very simply, a tritone substitution is when a Dominant 7th chord is replaced with the Dominant 7th chord a tritone (or 6 semitones or a diminished 5th) above it. For example, take this simple II-V-I progression in C major:
In the first four bars, the chord progression is played normally, but when it repeats, the G7 (or V) chord has been replaced with the one a tritone above it, the Db7 chord.
Why does tritone substitution work?
To understand why this works, lets have a closer look at the G7 and Db7 chords:
G7 = G B D F
Db7 = Db F Ab Cb(Cb is enharmonically equivalent to B)
As we have mentioned before in previous lessons, the most important notes in a chord, which give it its characteristic sound, are the 3rd and the 7th. In the G7 chord, the 3rd is the B and the 7th is the F. In the Db chord, the third is the F and the 7th is the B. So essentially these chords share the same 3rd and 7th degrees, only their places were swapped. This makes the chords interchangeable.
Why use the tritone replacement?
Using replacements in your playing makes you sound interesting and also adds different flavors to what you sound like. It also creates interesting new movements of chords, for example, the tritone substitution, when used in a II-V-I situation creates a nice chromatic bass movement created by the roots of the chords moving down (see the diagram above.)
Remember that the tritone substitution works in all keys, not just in C. Here is a list of some chords and their tritone replacements, the rest you can experiment and figure out yourselves: