Chord Substitutions

Chord substitution is replacing one chord with another that serves a similar harmonic function — either because they share important notes, or because they create similar tension and resolution patterns. Substitutions are how jazz musicians personalise the changes of a standard, and how composers create harmonic interest without abandoning the structure of the tune.

Tritone Substitution

The most important substitution in jazz. Any dominant seventh chord can be replaced by the dominant seventh chord a tritone (six half steps) away. The reason it works: both chords share the same tritone interval between their 3rd and 7th, just reversed.

G7 tritone sub = D♭7

G7: G B D F     (3rd=B, 7th=F   → tritone B↔F)
D♭7: D♭ F A♭ C♭  (3rd=F, 7th=C♭/B → tritone F↔B)

Both contain B and F — just swapped as 3rd and 7th.

In a ii-V-I in C:
Original:    Dm7 — G7  — Cmaj7
Tritone sub: Dm7 — D♭7 — Cmaj7

The D♭7 resolves by half step down to C — even stronger than G7.

The tritone sub bass movement (D♭ → C) is a descending half step — the smoothest possible resolution, smoother than the descending perfect fifth of G → C. This is why tritone subs sound so satisfying even though they seem to break the rules.

Diatonic Substitution

Chords that share most of their notes can substitute for each other:

In C major:
Cmaj7 ↔ Em7     (share E G B, differ on C vs D)
Cmaj7 ↔ Am7     (share C E G, differ on B vs A)
Fmaj7 ↔ Dm7     (share F A C, differ on E vs D)
G7    ↔ Bm7♭5   (both have dominant function, share B D F)

Substituting Em7 for Cmaj7 over a C pedal gives a softer, less definitive tonic. Substituting Am7 for Cmaj7 gives a minor colour to what should be major — creates momentary ambiguity.

Relative Minor/Major Substitution

Major chord → its relative minor (tonic area):
Cmaj7 → Am7     (Am7 is the vi of C major — relative minor)

Minor chord → its relative major:
Am7 → Cmaj7

These work in both directions as reharmonisation options.

Secondary Dominants

Insert a dominant seventh chord a fifth above any diatonic chord to create a temporary tonicisation:

Going to Dm7 in C major? Precede it with A7:
Standard: ... Cmaj7 — Dm7 ...
With secondary dominant: ... Cmaj7 — A7 — Dm7 ...

A7 is V7/ii (the dominant of Dm7).
It doesn't belong to C major but fits because it resolves strongly.

Diminished Seventh Substitution

A diminished seventh chord can substitute for the dominant seventh chord a half step below its root — it shares three of the four notes:

G7♭9: G B D F A♭
Bdim7: B D F A♭   (= same notes without the G root)

Bdim7 can substitute for G7♭9, especially for inner voice movement.

This is why diminished seventh chords appear so often as passing chords in jazz and classical music — they're dominant seventh chords in disguise, just viewed from a different bass note.

Reharmonisation in Chord Melody

These substitutions are the tools of reharmonisation — changing the chords under a fixed melody to create a new harmonic perspective on the tune. As long as the melody note works over the substitute chord (as a chord tone or acceptable extension), the substitution is valid. The most creative chord melody arrangements use all of these tools: tritone subs, diatonic subs, secondary dominants, and added passing chords.