Modulation changes an established musical key or tonal centre; reharmonisation changes the chords supporting a melody or passage. The techniques may occur together, but they describe different kinds of harmonic change.
Modulation versus tonicisation
A D7–G movement in a C-major passage can temporarily emphasise G, a process called tonicisation. If G then becomes a sustained point of arrival reinforced by bass, melody and cadences, the passage may have modulated to G major. A single chromatic chord alone is not sufficient evidence of a lasting key change.
Shared chords and pivot chords
C major has C, Dm, Em, F, G, Am and B diminished as its diatonic triads. G major has G, Am, Bm, C, D, Em and F-sharp diminished. Shared chords include G, Am, C and Em. Am can work as a pivot: vi in C major becomes ii in G major.
C major to G major
An illustrative progression is C–F–G–Am–D7–G, followed by further G-centred music. Am connects the two keys. D7 (D–F-sharp–A–C) supplies F-sharp, which distinguishes G major from C major; F-sharp can resolve up to G, while chordal C can descend to B. The continued G harmony helps establish the new centre.
Direct modulation
A piece can move abruptly to a new tonal centre, or use common tones, changed bass notes and other preparation rather than a formal pivot. Simply playing G after C does not prove a modulation if G is still perceived as the dominant of C. The later phrase must communicate what has changed.
Reharmonisation beneath a fixed melody
Consider four sustained melody notes E–D–C–B. Version A places Cmaj7–G–Am–G beneath them; Version B places Am7–G7–Fmaj7–G7 underneath the SAME notes. Each sustained note belongs to its chord in both versions, but the bass, harmonic direction and each melodic note’s role change.
One note, several harmonic meanings
E is the major third of C, the fifth of Am, the major seventh of Fmaj7 and the ninth of Dm9. Holding one pitch while changing harmony can create a different sound without any change to the melodic performance.
Reharmonisation and bass
Different chords may share important upper pitches but have different roots. A reharmonisation therefore includes deliberate decisions about bass motion and inversion, not just changing the chord labels. Chromatic substitutions can produce expressive tension, but their effect depends on the actual sounding melody and accompaniment.
Part of Volume 2 — How Music Works. Related topics: Borrowed chords · Chord progressions · Modal harmony · All How Music Works chapters.