Diatonic chord (Stufenakkord)
A chord built only from notes of the scale; every degree carries one.
What it means
A Stufenakkord is a chord consisting exclusively of notes within the scale and built on a particular degree of the scale. Every one of the seven degrees carries such a chord, which arises by stacking thirds from the stock of notes of the key. Diatonic chords are designated with Roman numerals.
Musical use
In major, three major, three minor, and one diminished triad result: major sonorities on the first, fourth, and fifth degrees, minor sonorities on the second, third, and sixth, a diminished sonority on the seventh. In natural minor the picture shifts accordingly; for the cadence, the seventh degree is raised there, whereby the dominant becomes a major sonority and the sonority on the seventh degree becomes a diminished triad.
Function theory interprets the diatonic chords according to their role. The main functions tonic (I), subdominant (IV), and dominant (V) stand on the major sonorities; the secondary degrees count as their representatives — the sixth as the tonic's relative, the second as the subdominant's relative, the third as the tonic's counter-chord or the dominant's relative, the seventh as an abbreviated dominant. Scale-degree theory describes the same sonorities by their numbers, without assigning them a function.
If the stacking is extended by a further third, diatonic seventh chords result on every degree — the normal case in jazz.
The diatonic chord is to be distinguished from the altered chord and from the secondary dominant, both of which contain notes foreign to the scale.
In scale-degree theory, every chord is defined by its degree: I, II, III, and so on. Large Roman numerals often designate major chords, small ones minor chords, a small circle the diminished triad. In jazz one speaks similarly of the diatonic chords of a key, usually formed as seventh chords. The degree designation is neutral with respect to function and is therefore also suited to modal music.
Performance practice
Play the diatonic chords of a key in succession and name them — that is the basis of every harmonizing. Listen to the alternation of major and minor between the degrees. And transfer the exercise into other keys; the sequence of chord types stays the same.