Chords in C major

C major is built from the notes C, D, E, F, G, A and B. It uses no sharps or flats at all. Harmonising that scale in thirds gives seven chords — 3 majors, 3 minors and 1 diminished — starting on C as the I. The chord on the fifth degree is G, a major V, so C major has the leading tone that makes a G-to-C cadence sound final.

Beyond those seven, C major commonly borrows Cm (i), Ddim (iiø), Eb (bIII), Fm (iv) and Gm (v) from the parallel minor — the mixture chords that give a progression somewhere unexpected to go.