Reharmonization became popular in the bebop era of jazz, according to Historyjazz.com. Although musicians had been reharmonizing melodies in previous forms of music, depending on the regional variations of certain songs, bebop was the first to consciously use it as an arranging and composing device. Since then, reharmonization has become a technique used in popular and rock music as well as jazz.
There are different types of reharmonization. Diatonic reharmonization, according to Malletjazz.com, uses alternate chords that stay within the key of the previous chord changes. Chromatic reharmonization uses chords that may not be in the original key of the song. Modal reharmonization borrows chords from specific modes, like dorian or phrygian, to create interesting harmonies to a melody.
To perform diatonic reharmonization, you will need to assign different chords from the existing key to the song you are playing. For instance, if you are in the key of C, the chords available to you will be C major, D minor, E minor, F major, G major, A minor, and B diminished. By replacing major chords with minor chords, you can create a more somber tone to your melody, and vice versa.
According to Apassion4jazz.net, chromatic reharmonization can use any chord in any key to replace existing chords in a song. Since this can be overwhelming, a good rule to follow is to use chords that have at least one chord tone in common with the chord being replaced. For instance, if you are replacing the chord Eb major, the notes in the existing chord are Eb, G, and Bb. Chords that share at least two of these tones are G minor, which consists of G, Bb, and D, C minor, which consists of C, Eb, and G, and Bbsus4, which consists of Bb, D, and Eb. Chords that share only one chord tone with Eb major are Eb minor, C major, and E minor.
Modal reharmonization concentrates on giving melodies a less-diatonic sound by eliminating IV and V chords from the key. In the key of C, the IV chord is F major, and the V chord is G major. By replacing these specific chords with reharmonizations, it destabilizes the key center, usually defined by the cadence of IV to I or V to I.