What is reharmonization?
Reharmonization is the process of finding new harmony for an existing melody. Each melody note can be interpreted as part of different chords. The note C can be the root of Cmaj7, the third of Am7, the fifth of Fmaj7, or the seventh of Dbmaj7.
Basic techniques
Three basic approaches: (1) diatonic substitution — replace a chord with another from the same key; (2) passing chords — insert chords between existing ones; (3) quality change — turn major into minor or dominant.
- Diatonic substitution: I → vi, IV → ii
- Passing chords: C → C#dim → Dm
- Quality change: Cmaj7 → Cm7
Advanced techniques
Tritone dominant substitution, ii-V chains, chromatic bass motion, pedal bass (one bass note while chords change above). Each technique can be combined. Jazz arrangers often use several techniques simultaneously.
The melody note rule
The main rule: the melody note must be a chord tone or an acceptable extension of the new chord. If the melody note is C, suitable chords include those containing C as root (C), third (Am), fifth (F), seventh (Db), ninth (Bb), etc.