Chords in A major

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

Beyond those seven, A major commonly borrows Am (i), Bdim (iiø), C (bIII), Dm (iv) and Em (v) from the parallel minor — the mixture chords that give a progression somewhere unexpected to go.