Functional Harmony
Chord substitution replaces one chord with another that has the same tonal function. The seven diatonic seventh chords belong to three functional families:
• Tonic (stable)
• Subdominant (semi-stable)
• Dominant (unstable)
Tonal function is determined by the tritone (6 half-steps, ♯4/♭5). In C major, the tritone is B–F (7-4). Chords are classified by whether they contain neither, one, or both of these notes.
Tonic Family
Chords: Cmaj7 (I), Em7 (iii), Am7 (vi)
These chords contain neither B and F together and are the most stable. Because they share the same tonic function, they can substitute for one another.
Examples:
• Cmaj7 → Em7 (up a major 3ʳᵈ)
• Cmaj7 → Am7 (down a minor 3ʳᵈ)
A Cmaj6 (C-E-G-A) can also substitute for Am7 (A-C-E-G) because they contain the same four notes.
Likewise, Cmaj7 can be substituted by Em7 because both produce a Cmaj9 sound when C is in the bass.
Subdominant Family
Chords: Dm7 (ii), Fmaj7 (IV)
These chords contain F (4) but not B (7). They share the same subdominant function and substitute freely.
Examples:
• Dm7 → Fmaj7 (up a minor 3ʳᵈ)
• Fmaj7 → Dm7 (down a minor 3ʳᵈ)
They share three common tones: F-A-C.
Dominant Family
Chords: G7 (V), Bm7♭5 (viiø7)
These chords contain both B and F, making them the most unstable chords in the key.
Example:
• G7 → Bm7♭5 (up a major 3ʳᵈ)
The dominant chord’s tritone (3-♭7) corresponds to the half-diminished chord’s tritone (1-♭5), giving both chords the same dominant function.
Tritone Substitution
A dominant seventh chord can be replaced by another dominant seventh chord whose root is a tritone away.
Example:
G7 ⇄ D♭7
Both chords contain the same tritone:
• G7 = B-F
• D♭7 = F-C♭(B)
The 3 and ♭7 simply exchange roles:
• G7: 3 = B, ♭7 = F
• D♭7: 3 = F, ♭7 = C♭(B)
Because they share the same tritone, both chords create the same dominant tension and resolve to the same tonic.
Example:
• G7 → Cmaj7
• D♭7 → Cmaj7
