Tritone substitution

Here is one of jazz's most elegant magic tricks. Take G7: its third and seventh, B and F, form a tritone. Now take D♭7 (D♭ F A♭ C♭): its third is F, its seventh C♭… which is B! The two chords share the same tritone, simply flipped.

Since those two notes create the tension and the resolution to C, you can replace G7 with D♭7: this is the tritone substitution (written SubV). The general rule: any dominant seventh chord can be replaced by the one a tritone (three whole tones) away.

M:4/4
L:1/4
%%staves {RH LH}
V:RH clef=treble
V:LH clef=bass
K:C
[V:RH] "Dm7"[Fc]4 | "G7"[FB]4 | "Cmaj7"[EB]4 | "Dm7"[Fc]4 | "Db7"[FB]4 | "Cmaj7"[EB]4 |]
[V:LH] D,4 | G,,4 | C,4 | D,4 | _D,4 | C,4 |]

Look at the right hand: it's identical in both versions! Only the bass changes. Instead of leaping D → G → C, it slides down chromatically: D → D♭ → C. That slide is part of what gives jazz its sophistication.

What it does to the tensions

The notes of D♭7 seen from G are altered tensions: D♭ = ♭5 (♯11), A♭ = ♭9, C♭ = 3. Playing D♭7 over a G bass therefore gives G7(♭9,♯11). Tritone substitution and the altered dominant are two sides of the same idea.

Variations

• Substituting secondary dominants: in A7 – D7 – G7 – C you can play A7 – A♭7 – G7 – D♭7 – C, and so on. The bass becomes chromatic.
• ii–V on the substitute: Dm7 – G7 can become A♭m7 – D♭7. Bolder.