The principle
Two dominant seventh chords a tritone apart share the same tritone between third and seventh.
- G7 = G B D F (tritone B–F)
- Db7 = Db F Ab Cb (Cb = B: same tritone F–B)
For this reason Db7 can replace G7 and resolve to C: it is called subV7 or bII7.
Effect
- The bass descends by semitone: Dm7 – Db7 – Cmaj7 instead of Dm7 – G7 – Cmaj7. A chromatic D–Db–C line, very smooth.
- Relative to G, the notes of Db7 are b5, 7, b9 and 3: it is equivalent to an altered G7 (b5, b9).
Where to apply it
- On the primary V: Dm7 Db7 C.
- On secondary dominants: A7 → Dm becomes Eb7 → Dm; D7 → G becomes Ab7 → G.
- With its related ii: Abm7 – Db7 – C.
- Chains: E7 A7 D7 G7 → E7 Eb7 D7 Db7 C (descending chromatic bass).
Caveats
- The melody must be compatible: a G in the melody over Db7 is the #11, fine; a D (the b9 of Db) is jarring.
- In a pop context the tritone substitution immediately sounds “jazz”: use it with intention (end of a bridge, last cadence).
Bridge with classical theory
Ab7 → G in C is enharmonically a German augmented sixth (Ab C Eb F#): the same sonority the classical composers used before the dominant.
Real example
In the A section of “The Girl from Ipanema” (Jobim, 1962), in F: Fmaj7 – G7 – Gm7 – Gb7 – Fmaj7. The Gb7 replaces C7, the dominant of F, and the bass descends G–Gb–F.