Primary chord

Three functions that carry a key

The page Diatonic chords calls them the three main functions of functional theory after Hugo Riemann: not every degree weighs equally. The tonic (T) on the first degree is home, the subdominant (S) on the fourth leads away from the tonic and opens up the space, the dominant (D) on the fifth creates tension and pushes back, especially strongly as a seventh chord.

The basic course T – S – D – T is the germ of almost every cadence. The page Circle of fifths calls the sequence I – IV – V – I the backbone of blues, rock, pop and country.

Why three chords are enough

The article One pattern predicts the chords of almost any pop song gives the reason: I, IV and V together contain all seven notes of the scale, so almost any melody in that key can be accompanied with them. Added to that is the sixth degree as the relative minor of the tonic, which can stand in for it.

The pentatonic depends on this too: the article The pentatonic as a first improvisation says that each of the five notes fits the primary chords of the key.

In minor

In A minor the primary chords are Am, Dm and E. According to the page Diatonic chords the dominant becomes E major in harmonic minor, so that the leading tone G♯ pulls to the tonic.