The idea
Replace a chord with another whose root is a third away (major or minor, above or below). There are two families:
- Diatonic: the substitute chord stays in the scale and shares 2–3 notes (see the card on diatonic substitution).
- Chromatic: the root moves by a third outside the scale (chromatic mediants, Coltrane changes, Tadd Dameron turnaround).
Diatonic substitutions by third (C major)
| Original | Third above | Third below |
|---|---|---|
| Cmaj7 | Em7 (E G B D) | Am7 (A C E G) |
| Dm7 | Fmaj7 | Bø7 |
| G7 | Bø7 | Em7 |
| Fmaj7 | Am7 | Dm7 |
Rule: the third below preserves the function better (Am7 for C, Dm7 for F); the third above produces a more "open" sound (Em7 for Cmaj7 = Cmaj9 without the root).
Chromatic substitutions by third
- I → bIII or bVI (C: Ebmaj7, Abmaj7): suspended endings, cinematic tag.
- Careful with dominants: a V7 is not substituted by a third (it would lose the tritone that defines it); it is substituted by tritone. By third you substitute arrival points (the local tonics) or insert new centers.
- Cycle of major thirds (Coltrane): C – Ab – E, each prepared by its V7.
- Cycle of minor thirds (octatonic axis): C – Eb – Gb – A: four centers that divide the octave into four; useful for modulating sequences and for "outside" improvisation.
The Tadd Dameron turnaround
Basic turnaround: | Cmaj7 A7 | Dm7 G7 |
- Tritone substitutions: | Cmaj7 Eb7 | Ab7 Db7 |
- The dominants become maj7: | Cmaj7 Ebmaj7 | Abmaj7 Dbmaj7 | → Cmaj7
The result I – bIII – bVI – bII is a descending-fifths movement (Eb → Ab → Db → C as a final half-step) with a mediant color. Dameron uses it at the end of Lady Bird.
Substitution by third in the bass (slash chords)
- C/E in place of Em: the bass is the third, the function remains tonic.
- Am/C = C6: the bass stays C while the chord moves a third below.
- F/A in place of Am: subdominant with the relative-of-tonic in the bass, much used in gospel.
Musical example (C major, melody G)
| Chord | Role of G | Result |
|---|---|---|
| Cmaj7 | 5 | neutral |
| Em7 | b3 | darker |
| Am7 | b7 | soft, "soul" |
| Ebmaj7 | 3 | bright and surprising (chromatic) |
| Abmaj7 | maj7 | dreamy (chromatic) |
The same melody note can be harmonized with five chords a third apart: it is one of the quickest techniques for reharmonizing a chorus.
Exercise
- Choose a long note in your melody.
- List all the chords a third away from the original chord (diatonic and chromatic) that contain that note as a chord tone or tension.
- Try each with a pad; keep the one that creates the right emotional color for the lyrics.
Related topics
Diatonic substitution, chromatic mediants, Coltrane changes, advanced reharmonization, harmony for film music.