22.39Chromatic approach dominant

Suppose target is Cmaj7.

You can approach it from one semitone above with:

D♭7 → Cmaj7

That is tritone substitution.

But you can also approach other chords chromatically:

Target Dm7:

E♭7 → Dm7

Target Gmaj7:

A♭7 → Gmaj7

This creates a general concept:

dominant chord one semitone above destination.

That dominant is usually the tritone substitute of the normal V.