In music, harmonization is the chordal accompaniment to a line or melody: "Using chords and melodies together, making harmony by stacking scale tones as triads".

A harmonized scale can be created by using each note of a musical scale as a root note for a chord and then by taking other tones within the scale building the rest of a chord.
For example, using an Ionian (major scale)

Using the minor (aeolian mode) one would have:

Reharmonization is the technique of taking an existing melodic line and altering the harmony that accompanies it. Typically, a melody is reharmonized to provide musical interest or variety. Another common use of reharmonization is to introduce a new section in the music, such as a coda or bridge.

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♭ augmented 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 composed of E♭ – F and G was originally harmonized with E♭maj7, 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.

In jazz, the term is typically used to refer to the process of reharmonizing some or all of a tune, whereby 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. Art Tatum was a pioneer of reharmonization, and later on John Coltrane, Miles Davis and Bill Evans were among the first to seriously explore its possibilities, and since then the technique has become an essential tool for the jazz musician and jazz arranger.