Key
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.
The seven chords
Extended to four notes each, the same seven degrees give Amaj7, Bm7, C#m7, Dmaj7, E7, F#m7, G#m7b5.
Borrowed chords
Capo 2 and play G major shapes to sound A major.
All eight capo positions for A major →