Tri-Tone Substitution theory and practice

Question

Can someone give me a practical explanation of the theory and practice of tri-tone substitution for jazz piano?

Answer 1

The basic premise of the tritone substitution is that we take a chord (typically a major-minor seventh) that has a tritone in it and substitute the entire chord with another major-minor seventh chord that has the same tritone.

Let's begin in C major and imagine a V7 chord that progresses to I. Within the V7 chord (G B D F) is the tritone B F. This is shown at letter A below.

As it turns out, this tritone exists in another major-minor seventh chord, this one on D♭: D♭ F A♭ C♭. (Note that the B F tritone is now enharmonically spelled as F C♭.) This is shown at letter B above.

So basically, wherever you use a G7 chord, you can substitute it with a D♭7 chord, because the tritone stays the same in both harmonies. Note, too, that the roots of the chords—G and D♭—are a tritone apart.

Listen to a recording here.

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We can take this one step further; let's stay in C major, but let's think about a secondary dominant to V. In other words, we're looking at V7/V, or D7. In this chord (D F♯ A C), the tritone is F♯ C; see C below. The tritone substitution of this harmony (again remembering that the roots are a tritone apart) will be A♭7 (A♭ C E♭ G♭) with the enharmonically spelled tritone C G♭; this is shown at letter D.

If you've learned about the German augmented-sixth chord (A♭ C E♭ F♯ in the key of C), you see here that it's enharmonically equivalent to the tritone sub of V7/V!