In music, reharmonization refers to the technique of taking an existing melodic line and altering the harmony which accompanies it. Typically, a melody is reharmonized to provide musical interest or variety. As well, reharmonization is often used to introduce a new section in the music, such as a coda or bridge.
Reharmonizing A Melody
A melodic tone can often be harmonized in a variety of different ways. For example, an E might be harmonized with an E major chord (E - G# - B). In this case, the melodic tone is acting as the root of the chord. That same E might be harmonized with a C major chord (C - E - G), making it the third of the chord. This concept extends to ninths (E would act as the 9th if harmonized with a Dm7 chord - D - F - A - C - E), fifths (E would act as 5 on an A major chord - A - C - E), and a wide array of other options.
Typically however, reharmonizations involve not just a single melody note, but a melodic line. As a result, there are often several melodic tones which might occur over a harmony, and all of these must be considered when reharmonizing.
For example, if a melody comprised of E - F and G was originally harmonized with Emaj7, choosing D7 as the reharmonization chord might not be the best choice, since each melodic tone would create semitone or minor 9th dissonance with chord members of the supporting harmony. Experienced arrangers might decide to use these kinds of highly dissonant chords when reharmonizing, however handling this dissonance requires a good ear and a deep understanding of harmony.
Jazz Reharmonization
In jazz, the term is typically used to refer to the process of reharmonizing an entire tune, where an existing melody is refitted with a new chord progression. Jazz musicians often take the melody from a well-known standard and alter the changes to make the tune sound more contemporary or progressive. John Coltrane, Miles Davis and Bill Evans were among the first to experiment with reharmonization in this way, and since then the technique has become an essential tool for the jazz musician and jazz arranger.
Chord Substitution
One of the most common techniques in jazz reharmonization is the use of substitute chords, through a technique known as tritone substitution. In tritone substitution, a dominant chord is replaced by the dominant chord a tritone away.
Tritone substitution works very well on standards, because the chord progressions typically utilize the II - V - I progression and the circle of fifths. For example, a jazz standard using a chord progression of Dm7 - G7 - Cmaj7 could easily be reharmonized to Dm7 - D7 - Cmaj7, (G7 is replaced with the dominant 7th chord a tritone away, D7). The new progression has a more contemporary sound, with chromatic bass motion and smooth voice leading in the upper parts.
Tritone substitution is also possible with major seventh chords, for example Dm7 - G7 - Cmaj7 could become Dm7 - Dmaj7 - Cmaj7. Thad Jones sometimes uses this type of substitution in his big band writing.
Chord Extension
In addition to using chord substitution, jazz musicians often reharmonize passages by adding notes to the original chordal framework. For example, a progression of Fm7 - B7 - E6 might be augmented to become Fm11 - B7(9) - E6/9. Here, the Fm7 chord (F - A - C - E) has been enriched by adding a G and B (the 9th and 11th). The B dominant seventh chord has had C added to it (the 9th), and the E chord an F (the natural 9th).
While there are a great many options for these types of reharmonizations, they are not always desirable, since the existing melody note may clash with the newly added pitches. Generally, reharmonization by adding notes works best when the new notes are diatonically derived.
Reharmonization with Quartal Harmony
Quartal harmony has been used by a number of jazz musicians, including Bill Evans, Miles Davis, and McCoy Tyner. Reharmonizing a melody with quartal harmony involves re-voicing the existing chords (which are usually based on tertian harmonies) so that the prevailing interval is that of a fourth (or its inversion, a fifth). This technique is particularly effective when the chords contain harmonic extensions.
For example, a Dm7 chord might be re-voiced as D - G - C - F. By extending the Dm7 chord to include the 11th, it is possible to maintain the interval of a fourth exclusively in the voicing.
When quartal harmony is used consistently in jazz harmony, it gives the music a strident, open and slightly dissonant sound.
Planing
Planing is a reharmonization technique used by both improvisers and arrangers. It refers to the technique of sliding a chord (or chord tone) up or down chromatically, maintaining the shape and voicing of the chord. For example, F7 (F - A - C - E) could slide up to become G7 (G - B - D - F). Each note has been "planed" up a semitone.
Planing is often used by jazz arrangers to reharmonize melodic passing tones which, if voiced as a vertical sonority, might clash with the prevailing harmony in the progression. As well, a number of improvisers have used planing effectively, typically as part of a progression. Herbie Hancock uses improvised planing on his tune "Chameleon", on his 1973 Headhunters record. (Courtesy of Wikipedia)
