Chord substitutions — the tritone sub

Concept

The tritone substitution: replace a dominant 7 chord with another dominant 7 a tritone (6 frets) away. They share the same 3rd and 7th. So in a ii-V-I, instead of Dm7-G7-Cmaj7, play Dm7-Db7-Cmaj7. The bass moves chromatically down — a powerful jazz move.