How would you define a tritone substitution?

Tritone substitution works because the two chords share two pitches: namely, the third and seventh (reversed).

In a G7 chord, the third is B and the seventh is F; whereas, in its tritone substitution, D♭7, the third is F and the seventh is C♭ (enharmonically B♮).

Notice that the interval between the third and seventh of a dominant seventh chord is itself a tritone.

Tritone substitution is a popular reharmonization method, where the bass note of a Dom7 V chord (can be the main dominant or any auxiliary dominant before any chord) gets shifted a tritone away.

The 3-b7 of the chord that create the release tension of the Dominant chord remain the same (eg. G7 with B-F becomes Db with F-Cb).

When altering a chord, the altered notes all remain functional as well (eg. G7#9b13 with Bb, Eb extensions becomes a Db13). Y

ou can truly “substitute” the chord but also very popular is to use both in conjunction, meaning to add the tritone sub additionally to add anticipation.