Reharmonisation basics

Reharmonising means replacing a melody's original chords with others, richer or more surprising, while keeping the melody intact. It's one of the jazz pianist's great arts: Art Tatum, Bill Evans or Herbie Hancock turn a simple song into a harmonic landscape.

The golden rule

The melody note must be a chord tone or an available tension (9, 11, 13 depending on the chord type) of the new chord, at least on strong beats. After that, anything goes.

Six techniques, from simplest to boldest

1. Enrich: replace triads with seventh chords and extensions (C → Cmaj7 or C6/9).
2. Diatonic substitution: replace a chord with one of the same function (C → Am7 or Em7; F → Dm7).
3. Add ii–Vs: precede each target chord with its own ii–V (Dm7 – G7 before C; Em7 – A7 before Dm).
4. Secondary dominants and tritone substitutions, seen earlier.
5. Passing diminished chords: C – C♯°7 – Dm7 links two chords with a chromatic bass.
6. The “backdoor” ii–V: Fm7 – B♭7 – C (iv7 – ♭VII7 – I), a sideways resolution, soft and nostalgic.

An example

Let's go back to “Au clair de la lune”. The original version:

M:4/4
L:1/4
K:C
"C"c c c d | "C"e2 "G"d2 | "C"c e "G7"d d | "C"c4 |]

And a reharmonisation, with shells in the left hand:

M:4/4
L:1/4
%%staves {RH LH}
V:RH clef=treble
V:LH clef=bass
K:C
[V:RH] "Cmaj7"c c "Am7"c d | "Dm7"e2 "G7"d2 | "Fmaj7"c e "Bb7"d d | "Cmaj7"c4 |]
[V:LH] [C,B,]2 [A,,G,]2 | [D,C]2 [G,,F,]2 | [F,,E,]2 [_B,,_A,]2 | [C,B,]4 |]

Let's check the golden rule: C is the third of Am7 and D its 11th; E is the 9th of Dm7; C and E are the fifth and seventh of Fmaj7; D is the third of B♭7 (the “backdoor” chord leading to C). Everything holds together, and the nursery tune takes on the colour of a ballad.