Tritone substitution and chromatic dominant movement

The interval that both chords share

G7 contains B and F. Db7 contains F and Cb, which is the same note as B. The two chords are built on roots a tritone apart, and they contain the same tritone — with the third and seventh swapping roles. Since the tritone is what makes a dominant pull, the two chords pull the same way.

What changes is the bass. G7 to Cmaj7 drops a fifth. Db7 to Cmaj7 slides down a semitone. Put a Dm7 in front and the bass walks D, Db, C — three consecutive semitones where before there were two leaps. That descending line is the whole reason the substitution became standard practice.

How it is written

The usual notation is subV7, and when the substitution targets something other than the tonic it takes a slash: subV7/ii, subV7/V. Some books write bII7 instead, which describes the chord’s position rather than its job. This analyzer uses subV7, because the point of the chord is what it is standing in for.

How the analyzer decides

A dominant seventh is read as a tritone substitute when the chord a semitone below it is a degree the key can tonicize, and the reading is strengthened when the progression actually goes there. If it does not resolve down a semitone, the analyzer will usually prefer another explanation and say so — a dominant that resolves nowhere is more often a borrowed chord or a chromatic approach than a substitution nobody completed.

Compare the two examples at the top of this page: Dm7 G7 Cmaj7 and Dm7 Db7 Cmaj7. The second gets labelled as a ii-V-I with the dominant substituted, and the relationship between Db7 and Cmaj7 is explained as a semitone slide carrying the tritone of the chord it replaced.

Hear it in your own progression

Paste a progression of your own, or load one of these, and see where the device shows up.