A Major
0 - 4 - 7Root + major 3rd + perfect 5th
About A Major
A major is three fingers in a row at the second fret — the tidiest shape on the guitar, and the one most often played wrong. Cramped into one fret space, the fingers mute the high E string almost as a rule, and a beginner hears a chord that is nearly right.
The notes
A, C♯, E — root, major third, perfect fifth. The open A string gives the root and the open high E the fifth, so the three fretted notes fill in the middle. The chord has five sounding strings; the low E is left out.
Move the C♯ down one fret and it becomes A minor. Both chords share the same two open strings, which is why they sound related in a way that, say, A and B♭ do not.
The fingering problem
Three fingers on adjacent strings at the same fret is the tightest arrangement in first position. Players solve it in three ways: index-middle-ring in order, a small barre with one finger across three strings, or middle-ring-little to keep the index free.
None is more correct. The barre is fastest for changes, the three-finger version is cleanest for beginners, and the choice usually depends on what chord comes next.
Where it shows up
As tonic of its own key, the IV of E major and the V of D. All three are guitar keys, and A sits at the centre of them: a huge share of blues and rock lives inside the triangle A, D and E.
Lift a finger and you get Asus2; add one and you get Asus4. Both are used constantly, because the shape leaves room for them and because a plain A held for four bars grows dull fast.