A tritone substitution is the substitution of one chord with another chord whose root is a tritone (i.e., three whole steps) away from the root of the original chord.
For example, if you have a C7 chord (C E G Bb), a tritone substitute could be a Gb7 chord (Gb Bb Db Fb).
One of the reasons that tritone substitutes can be effective is that they share two common notes. The third and seventh of the C7 chord are E and Bb. Those same notes are the seventh and third of the Gb7 chord.
Tritone substitutions can provide nice chromatic movement. For example, look at a typical I-vi7-ii7-V7 progression:
CMaj7 | Amin7 | Dmin7 | G7 | CMaj7
If you use a tritone substitute for the V7 chord, you get the following progression:
CMaj7 | Amin7 | Dmin7 | Db7 | CMaj7
I'm sure that others can provide a more thorough explanation, but I hope this helps.
Last edited by Febs : 04-30-2010 at 03:03 PM.