Diatonic (scale tone)
Notes or chords formed exclusively from the degrees of the underlying scale; opposite: chromatic/non-diatonic.
What it means
A note or chord is diatonic when it is built entirely from the seven degrees of the underlying scale, without any chromatic alteration. In C major these are the notes C, D, E, F, G, A, B and every triad or seventh chord formed exclusively from them, such as the dominant seventh chord G-B-D-F.
Musical use
The distinction between diatonic and non-diatonic sonorities underlies scale-degree theory and helps describe the harmonic frame of a diatonic passage. Diatonic should be distinguished from non-diatonic (chromatically altered) sonorities, which contain at least one note outside the underlying scale.
Performance practice
When practising analysis, mark diatonic chords first, so that chromatic, non-diatonic sonorities stand out more clearly as deliberate harmonic departures.