In tonal music theory, a diatonic function (also chord area) is the specific, recognized role of each of the 7 notes and their chords in relation to the diatonic key. In this context, role means the degree of tension produced by moving toward a note, chord or scale other than the tonic, and how this musical tension would be eased (resolved) towards the stability of returning to the tonic chord, note, or scale (namely, function).

Three general and inseparable essential features of harmonic function in tonal music are:

A fourth feature is the ambiguity that arises from the use of the same terms to describe functions across all temporal spans of a hierarchical structure from the surface to the deepest level, and that the longer term or deeper functions act as a center for shorter higher level ones and that the functions of each tend to counteract each other. "Harmonic function essentially results from the judgment that certain chords and tonal combinations sound and behave alike, even though these individuals might not be analyzed into equivalent harmonic classes," for example V and VII. "Harmonic function is more about...similarity than equivalence".