nondiatonic

Nondiatonic refers to notes, chords, or harmonies that fall outside the diatonic collection of pitches defined by a given key. In tonal music, the diatonic set consists of the seven notes of the key’s major or natural minor scale; any pitch not in that set is nondiatonic relative to the key.

Nondiatonic material can appear within a diatonic context as chromatic passing tones or other nonharmonic tones that momentarily create dissonance before resolving to diatonic tones. It also appears as part of chords or progressions that are not built entirely from the diatonic scale. When used, nondiatonic elements typically add color, tension, or direction toward a cadence or a modulation.

Common kinds of nondiatonic harmony include modal mixture (borrowed chords from the parallel key), secondary dominants (V of a diatonic chord), chromatic mediants, diminished seventh chords, augmented sixth chords, and tritone substitutions. In classical music, nondiatonic passages are carefully integrated to preserve voice leading and functional coherence; in jazz and contemporary music, nondiatonic harmony and chromaticism are broader and more freely deployed.

Understanding nondiatonic material involves recognizing its relationship to the surrounding diatonic framework; a pitch may be nondiatonic in one key but diatonic in another when the tonal center shifts. Related concepts include chromaticism, modal mixture, borrowed chords, and secondary dominants.