diatonic

Diatonic describes notes, scales, and chords that belong to a key or to the seven-note family derived from the major scale. In this sense, a diatonic set consists of seven pitch classes within an octave, and melodies and harmonies typically stay within that collection.

The diatonic scales include the major scale (Ionian) and its relative natural minor (Aeolian), together with the five modes generated by rotating the same seven notes: Dorian, Phrygian, Lydian, Mixolydian, and Locrian.

Diatonic harmony uses chords formed by stacking diatonic thirds from those notes, producing triads such as I, ii, iii, IV, V, vi, and vii diminished. This diatonic harmony underpins much of Western tonal music, providing functional relationships among tonic, predominant, and dominant areas.

By contrast, chromaticism involves notes, chords, or melodic elements outside the diatonic collection, including borrowed chords and alterations. The term diatonic originates from Greek diatonos, meaning through tones, reflecting its focus on scale steps within the key.