Diatonic means of, or using, only the seven notes of a standard major scale or natural minor scale without chromatic variations; diatonic literally means "from the key". So, diatonic scales are those of the major and minor keys, and diatonic passages, intervals, chords, and harmonies are those made up of the notes of the key prevailing at the time. The modes are also considered to be diatonic.

In a diatonic scale there are five whole steps and two half-steps, arranged so that the two half-steps are separated from each other by either two or three whole steps. This arrangement ensures that in a scale spanning more than one octave, all the half-steps are separated by the greatest possible interval from each other – at least two whole steps.