Chords in G major

G major is built from the notes G, A, B, C, D, E and F#. Its scale carries 1 sharp. Harmonising that scale in thirds gives seven chords — 3 majors, 3 minors and 1 diminished — starting on G as the I. The chord on the fifth degree is D, a major V, so G major has the leading tone that makes a D-to-G cadence sound final.

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