Chord Substitution: Diatonic, Secondary Dominants, and Tritone Subs

Chord substitution means replacing one chord with another that serves the same harmonic function. The substitute preserves where the progression wants to resolve while changing its color. The three most common techniques are diatonic substitution (swap a chord for a key-relative that shares notes, like Am for C in C major), secondary dominants (precede any chord with its V to add chromatic tension), and tritone substitution (replace a dominant 7th with another dominant 7th a tritone away, the jazz reharmonization move).

A tritone substitution replaces a dominant 7th chord with another dominant 7th exactly a tritone (6 semitones) away. G7 can be replaced with D♭7 (or C♯7); the two chords share their 3rd and 7th (B/F and F/B, just swapped). In a ii–V–I in C major (Dm7–G7–Cmaj7), the tritone sub produces Dm7–D♭7–Cmaj7 — the chromatic descending bass (D–D♭–C) is the signature sound of jazz reharmonization.

A secondary dominant is a dominant 7th (or major triad) chord that temporarily tonicizes a non-tonic chord. In C major, E7 is the secondary dominant of Am (V/vi) — it creates a strong pull toward Am even though E7 is not diatonic to C major. Secondary dominants let you chromaticize any chord in the key by preceding it with its own V. They are everywhere in jazz, Broadway, soul, and pop bridges.

Diatonic substitution replaces a chord with another chord from the same key that shares notes and serves the same function. In C major, the tonic function is shared by C, Am, and Em (all contain the tonic notes C and E). So Am or Em can substitute for C without changing key. Similarly F and Dm share subdominant function. Diatonic subs are the gentlest substitution — they recolor without adding any chromaticism.