A scale provides possible pitches; the currently sounding chord determines how each pitch functions. Chord–scale relationships help composers and improvisers distinguish chord tones from passing notes, extensions and deliberate outside pitches.
Chord tones versus scale tones
C major contains C–D–E–F–G–A–B. Cmaj7 contains only C–E–G–B. D, F and A are still in the key but outside the named four-note chord. D can function as a ninth, A as a thirteenth and F as a natural eleventh, which may rub against E in a tight voicing.
Guide tones in ii–V–I
Dm7 (D–F–A–C) has F as third and C as seventh. G7 (G–B–D–F) has B as third and F as seventh. Cmaj7 (C–E–G–B) has E as third and B as seventh. F changes from third to seventh between Dm7 and G7, while B changes from third of G7 to seventh of Cmaj7.
Melodic targets
A melody can outline a changing progression by emphasising thirds or sevenths as each chord arrives. Over Dm7–G7–Cmaj7, the target sequence F–B–E outlines the thirds of the three chords. Adding passing notes such as E before F, A before B and D before E provides contour and rhythmic movement.
Extensions and approach notes
A over G7 is a ninth and E can be heard as a thirteenth. A short B-flat approaching B over G7 is a chromatic approach outside C major, but need not signal a modulation. Note length, accent and surrounding harmony influence how strongly a passing or colour note is perceived.
Modal chord–scale labels
Dm7 can be described with D Dorian, G7 with G Mixolydian and Cmaj7 with C Ionian. In the key of C, all three scales share the same seven pitch classes. Their labels help analyse notes relative to each chord, but do not necessarily indicate that the overall tonal centre changes every bar.
Changing the chord beneath a fixed note
B natural belongs to Cmaj7 but not to C7, whose minor seventh is B-flat. Holding melodic B while switching the accompaniment from Cmaj7 to C7 creates a changed harmonic relationship. It may be deliberately tense or lead to a melodic revision.
A practical comparison
Keep a Dm7–G7–Cmaj7 accompaniment unchanged. First write a melodic line primarily using chord-tone targets, then make a second version with ninths, passing tones and one controlled chromatic approach. Listening to both versions reveals how a scale becomes musical phrasing rather than a fixed list of permitted notes.
Part of Volume 2 — How Music Works. Related topics: Seventh chords · Chord extensions · Modal harmony · All How Music Works chapters.