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Each major and minor key contains seven diatonic triads built from its scale degrees. In major keys, triads are I (major), ii (minor), iii (minor), IV (major), V (major), vi (minor), and vii° (diminished). In minor keys, the diatonic chords vary depending on whether natural, harmonic, or melodic minor is used. The harmonic minor scale (with its raised seventh) provides a strong V chord natural to the key. Learning diatonic chords enables composition and analysis within keys without accidentals.
Build all diatonic triads in multiple major and minor keys. Analyze progressions in pieces to identify how composers use diatonic chords.
From your work with diatonic triad harmonization, you know how to build a triad on any scale degree. Now the goal is to internalize the complete family of chords that lives inside a single key — the diatonic chord system. Think of it as the palette a composer in a given key is working with: seven chords, each with a characteristic quality and a characteristic function, and together they define what "staying in the key" means.
In major keys, the seven diatonic triads follow a fixed pattern of qualities: I (major), ii (minor), iii (minor), IV (major), V (major), vi (minor), vii° (diminished). The uppercase Roman numerals indicate major triads, lowercase indicate minor, and the degree symbol indicates diminished. In C major: C major (I), D minor (ii), E minor (iii), F major (IV), G major (V), A minor (vi), B diminished (vii°). These qualities are not arbitrary — they fall out directly from the major scale's interval pattern. You don't need to memorize them as disconnected facts if you remember the scale structure and know your triad intervals.
Minor keys introduce a complication because there are three versions of the minor scale, each producing different chord qualities on some degrees. Using natural minor (the unaltered descending form), the diatonic triads in A minor are: i (minor), ii° (diminished), III (major), iv (minor), v (minor), VI (major), VII (major). Notice that the v chord in natural minor is *minor*, not major — this matters enormously for harmonic strength. The dominant chord V derives its powerful pull toward the tonic from the leading tone (the note a half step below the tonic). In natural minor, that note is a whole step below (the subtonic), so the "dominant" chord lacks the leading tone's gravitational pull. Harmonic minor fixes this by raising the seventh scale degree, which converts the v chord into a major V — restoring the strong dominant-to-tonic resolution. This is why harmonic minor exists as a separate scale variant: it exists to provide a functional V chord in minor keys.
The practical consequence is that when analyzing or composing in a minor key, you must specify which scale degree is being used for chords built on scale degrees involving the 6th and 7th. The chord on scale degree VII might be B-flat major (natural minor) or B diminished (harmonic minor) in A minor. Composers mix freely — using natural minor for melodic lines and harmonic minor for cadential chord progressions — which is why minor-key music sounds richer and less predictable than major-key music. The vii° chord (diminished triad), present in both major and harmonic minor, almost always appears in first inversion (the third of the chord in the bass), where its characteristic tension resolves most smoothly into the tonic chord.
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