22.8Tritone substitution formula

A dominant chord can often be replaced by another dominant chord whose root is a tritone away.

G ↔ D♭

D ↔ A♭

A ↔ E♭

E ↔ B♭

C ↔ G♭

F ↔ B

So:

G7 → C

can become:

D♭7 → C

D7 → G

can become:

A♭7 → G

A7 → D

can become:

E♭7 → D