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Everything said about scales so far concerned melody: a scale is a selection of pitches. This entry covers the other half: a scale also decides the harmony.
One step is all it takes: stack thirds above each note of the scale (take every other note) and a set of chords appears.
The seven triads on a major scale
| Degree | 1 | 2 | 3 | 4 | 5 | 6 | 7 |
|---|---|---|---|---|---|---|---|
| Quality | major | minor | minor | major | major | minor | diminished |
| Roman | I | ii | iii | IV | V | vi | vii° |
This table is the complete inventory of diatonic chords, and it accounts for almost every basic conclusion in harmony:
- I, IV and V are major → so tonic, subdominant and dominant form the tonal skeleton;
- only vii is diminished → so it is unstable and must resolve;
- vii's root is the seventh degree (the leading tone) → so it points at I, which is where cadences come from
(see cadence).
None of this is a rule imposed from outside; all of it follows from the scale's structure. Change the structure and the chord sequence changes with it — which is why each mode has its own harmonic colour.
Change the scale, change the whole chord set
| Scale | Chord on degree 1 | Character |
|---|---|---|
| major | major | stable, bright |
| natural minor | minor | dark, and its dominant is minor → weak pull (hence harmonic minor) |
| [[concept:dorian | Dorian]] | minor |
In other words: saying how a mode sounds is half saying which chords can grow on it.
Diagram: stacking thirds
Listen: an arpeggio is a scale stood on end
An arpeggio plays three notes of the scale in turn — think of it as tipping the scale upright. It is the shortest path connecting melodic material to harmonic material.
Common misconceptions
- "A scale is only about melody." It also fixes which chords are available; change the scale and the whole harmony changes.
- "Diatonic chords are a set of rules." They are the necessary result of stacking thirds.
- "A minor key's dominant is a minor triad." True of natural minor, but in practice the seventh is almost always raised to give a major dominant (harmonic minor), or cadences will not hold.
- "Every mode has the same chord sequence." They are all different. Dorian and natural minor differ by one melodic note but Dorian gains a major triad where minor has a minor one.
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Listen in the library
All tracks from midicn-lib — inline badge is the license tier (C1 commercial / C2 non-commercial / C3 study only).
Sources
- 在音阶各音上叠三度即得调内三和弦（大调为 大三/小三/小三/大三/大三/小三/减三），属和声学通则
- 调式各音上叠三度所得和弦序列不同，为调式和声学的通行表述
Used for fact-checking only; all prose is original.