Progressions: I IV V vi

Key idea Chords are numbered by scale degree. In C major: I = C, IV = F, V = G, vi = Am. The I V vi IV loop (C G Am F) runs through thousands of songs.

Build a triad on each degree of the scale and number it with Roman numerals. Upper case is major (I, IV, V), lower case is minor (ii, iii, vi). Degree 7 is diminished (vii°).

The I V vi IV loop

Changing key just changes the names; the numbers stay. In G major, I V vi IV is G, D, Em, C.

• I IV V I (C F G C): folk songs, children's songs, blues.
• vi IV I V (Am F C G): the same four chords, starting dark.
• I vi IV V (C Am F G): 1950s songs and many ballads.
• ii V I (Dm G C): the backbone of jazz.