Ah, something I've seen lately but which hasn't quite sunk in yet- Tritone substitution. Care to give a quick overview?
OK I'll give it my best shot. Tritone substitution is a little trick that jazz composers use to sneak chords in to a song that are not strictly speaking in the right key. The word Tritone means 3 tones (6 semitones or frets away). Other names for the Tritone are b5 or the devil's interval. The Tritone of C is Gb and of B is F and so forth
Lets use the key of C for our example. The chords that perfectly match the key of C in 4 part harmony are Cmaj7 Dm7 Em7 Fmaj7 G7 Am7 Dm7(b5). G7 is known as the dominant chord in C. G7 is a good place to make the tritone substitution (why? see *). The Tritone substitution for G7 is Db7. So if you have a chord progression in C e.g. Dm7 / G7 / Cmaj7 the Tritone substitution would be Dm7 / Db7 / Cmaj 7
* The individual note of the G7 chord are G B D F = (1 3 5 b7). B and F are a Tritone apart. Tritone intervals are still Tritones when inverted so swop B and F around - ? F ? B = (1 3 5 b7). Answer Db F Ab B = (1 3 5 b7) which are the notes in the Db7 chord. PHEW!!