Chords in a key and Roman numerals

Build a triad on each of the seven notes of any major scale and the pattern is always the same: three major, three minor and one diminished.

Seven triads from one scale

Take the C major scale and build a triad on each note using only notes of the scale. On C you get C E G, major. On D, D F A, minor. On E, E G B, minor. On F, F A C, major. On G, G B D, major. On A, A C E, minor. On B, B D F, diminished. These are the diatonic triads of C major, the chords that belong to the key. Because every major scale has the same pattern of steps, every major key has the same pattern of qualities: major on 1, 4 and 5; minor on 2, 3 and 6; diminished on 7.

Roman numerals

Musicians label these chords with Roman numerals for the scale degree of the root, and the case of the numeral shows the quality. Uppercase means major (I, IV, V), lowercase means minor (ii, iii, vi), lowercase with a small circle means diminished (vii°), and uppercase with a plus means augmented. In any major key the row reads I, ii, iii, IV, V, vi, vii°. The numeral describes the chord's job in the key, so V is the dominant chord whether the key is C (G B D) or E♭ (B♭ D F).

The three major chords, I, IV and V, are called the primary chords. Between them they contain every note of the scale: in C major, C E G, F A C and G B D cover C, D, E, F, G, A and B. That is why many folk songs and hymns can be accompanied with just these three chords. On the circle of fifths they are neighbours: IV sits one step counterclockwise from I and V one step clockwise.

Triads in minor

In natural minor the same process gives i, ii°, III, iv, v, VI, VII. In A minor: Am, B°, C, Dm, Em, F, G. Composers usually raise the seventh degree (G to G♯) when it is part of a dominant chord, to get a leading tone, so v (E G B) becomes V (E G♯ B) and VII (G B D) becomes vii° (G♯ B D). The usual row in minor is therefore i, ii°, III, iv, V, VI, vii°. A raised seventh inside the III chord would make it augmented (C E G♯), so that chord normally keeps the natural seventh.

To write a key and its chords, analysts put the key name first (C: for C major, a: or Am for A minor) and the numerals under each chord. Reading a progression as numbers instead of letters makes it portable. I-IV-V-I in C is C, F, G, C; the same numbers in D are D, G, A, D. A singer who needs a lower key can ask for the progression in another key without anyone rewriting the chord chart from scratch.