The tritone substitution is a powerful technique both for chordal playing and improvisation. It makes use of an interesting harmonic phenomenon. In any dominant 7th chord, the two most important notes (the 3rd and the 7th) form the interval of a tritone. In the case of C7, the 3rd is E and the 7th is Bb. But what if you were to make E the 7th and Bb the 3rd? You would essentially turn the interval upside down and make a new chord:
The above example shows a C7 going to a Gb7 with Db in the bass. Notice that the Bb and the E appear in both chords. That shows the tritone substitution in action. Also the roots of the two chords are a tritone apart. The two chords can function as the same chord, as shown here:
This has enormous implications for improvising. You can choose scales, arpeggios and licks based on the tritone sub. Here's an example:
Instead of G7 and Db7 above, try playing the lick over Dmi7 and G7b9 and see what you get. In other words, a lick based on a tritone sub sounds especially effective over an altered chord. Here is a line based on the D dorian and Db Mixolydian (tritone sub of G Mixolydian) scales. Check it out.
You can see how the tritone substitution can open huge areas of exploration for your improvising. Try it out and we'll be back soon with more examples.