Diatonic harmony is the system of chords built from the notes of a scale, using only the pitches that belong to a given key. Every note in a scale can serve as the root of a chord, and the intervals available within the scale determine each chord's quality. This produces a fixed set of chords for each key—the diatonic chords—that form the harmonic vocabulary of tonal music.
To build a diatonic chord, start on any scale degree and stack notes in thirds, skipping every other scale note. In C major, starting on C and stacking thirds gives C-E-G: a C major chord. Starting on D gives D-F-A: a D minor chord. The scale determines the intervals, and the intervals determine the chord quality.
Building a Triad on the First Degree
Stacking thirds from C in the C major scale produces C-E-G, a major triad
This process yields seven triads in any major key—one rooted on each scale degree. Because the spacing between scale notes varies (some thirds are major, some are minor), the resulting chords have different qualities. This is not arbitrary; it emerges directly from the interval structure of the major scale.
Musicians label diatonic chords with Roman numerals corresponding to their scale degree. Upper-case numerals indicate major chords, lower-case indicate minor, and a degree symbol (°) marks diminished chords. This system lets you describe chord progressions independently of any specific key.
The seven diatonic triads in a major key:
In C major, this produces:
This notation is powerful because a progression like I-IV-V-I describes the same harmonic motion whether you're in C major, F major, or any other key. It separates the function of a chord from its specific pitches.
Natural minor produces a different set of chord qualities because its interval pattern differs from major. The seven diatonic triads in a natural minor key are:
In A minor:
Notice that the minor dominant (v) lacks the strong pull toward the tonic that the major dominant (V) provides. This is why harmonic minor raises the seventh degree—it creates a major V chord with a leading tone that resolves convincingly to the tonic.
Adding a fourth note (another third on top of each triad) creates diatonic seventh chords, which add richness and specificity to harmony. In C major, the diatonic seventh chords are:
Seventh chords are essential in jazz, where nearly every chord in a progression uses the seventh. In pop and classical music, seventh chords appear more selectively—often on the dominant (V7)—to add color or increase harmonic tension at key moments.
Understanding diatonic harmony reveals why certain chords sound "right" together. When you play C major, F major, and G major in sequence, they feel coherent because all three are diatonic to the same key. Introducing a chord from outside the key—like Bb major—creates surprise precisely because it breaks the diatonic expectation.
Diatonic harmony also explains common chord progressions. The ubiquitous I-IV-V and I-vi-IV-V progressions aren't random discoveries; they emerge from the relationships between diatonic chords. The dominant (V) resolves to the tonic (I) because of the leading tone and tritone within the V chord. The subdominant (IV) moves naturally to V because of voice leading tendencies. These patterns are explored further in functional harmony and chord progressions.