Watch the Chord Substitution: 4 online guitar lesson by TrueFire from Jazz Guitar Greatest Hits Vol. 2
Lesson Source: Fareed Haque's Bebop Improv Survival Guide
Principle 4: TriTone Subs!
This one seems really hairy, sounds awesome, but actually is pretty simple. Imagine a bass or tuba player back in the day. playing G,G,G,G - C,C,C,C - G,G,G,G TO C,C,C,C again and agan and again. Ach! Someone shoot me quick! Thats what any bored bass player or tubist [tubist?] would be saying after 200 gigs. Eventually, that bored tubist played G,G,G,G TO C,C,C,C TO G,G THEN CHROMATIC Db,Db !!! TO C.
That little step for a tuba player, one giant leap for all of JAZZ HISTORY. 'Cuz when you play a G7 with a Db in the bass, justa chromatic little note leading back home, tghat Db creates a Db7 b5 chord. So with that little chromatic move, the whole chord was reinvented and a sound was created that would define the sound of BeBop and modern jazz. So dm7-G7-Cmaj7 could become D7-G7-C7 and then become D7-Ab7-G7-Db7-C7.