What is tritone substitution? (my own explanation)

I was in a meeting with Kenny the other day and he asked me on the spot what tritone substitution was.

The answer I gave from the top of my head was something like this: 

Tritone substitution is a method of chord substitution, mostly used in jazz harmony.  It can work in two ways: 1) substituting the notes of a dominant seventh chord with the major triad of the related tritone to the root chord (eg. replacing a G7 with a Db7 in the right hand while keeping the G in the left hand on the piano) or 2) replacing the bass/root note of a chord with the bass/root note of the tritone relevant to that key (eg. G7 would become G7/Db).  The former method is the one more commonly used.

Tritone substitution can be very dissonant and must be used in the correct context in order for it to sound good.  However, it works because tritone dominant 7th chords share the guide tones (3rd and 7th) - only they're flipped around.  So for example, G7 has B as its 3rd and F as it's 7th.  Db has F as its 3rd and B as it's 7th.  This means the only notes that change are the fifth and root. If you have a G7 chord and replace the G triad with a Db7 triad, the fifth becomes flattened, along with the 9th, altering the chord.  Again, this can sound horrible if not within the right context - it almost always works in a ii-V-I setting.  Why?  Because in a ii-V-I setting, the tritone substitution is implying a chromatic run down.  If you have a ii-V-I in the key of C, you have Dm7-G7-Cmaj7.  Tritone sub the G7 for Db and you have Dm7-Db7-Cmaj7.  The chord moves down chromatically from D to C, giving a sense of harmonic movement and purpose.  There is tension, but it's instantly released as the guide tones of the substituted chord move down to resolve into the C.

Phew!

Tritone substitution is a very important part of the ruleset.  I'll soon be uploading a section of the rules  based on chord substitution - the tritone based rules can be found there.

J