A Minor
m
0 - 3 - 7Root + minor 3rd + perfect 5th
About A Minor
A minor is the chord with no sharps and no flats — the plainest minor there is. On guitar it takes three fingers in first position and shares two of them with E major, which is why the two chords sit next to each other in so many songs: the hand barely moves.
The notes
A, C, E — root, minor third, perfect fifth, all natural notes. That is what makes A minor the reference minor the way C major is the reference major: it is the key signature with nothing in it, the one written on a blank stave.
The chord is the relative minor of C major and shares two of its three notes. Play C then Am and almost nothing changes underneath — only the bass moves from C to A — yet the mood turns completely. It is the cheapest emotional shift available on the instrument.
The shape and its neighbours
The first-position shape is E major moved one string across. Learn one and you have most of the other, which is why beginners are usually taught them together.
From A minor, one finger gives you Am7 (lift the third finger) and another gives Asus2. Both are common in songs that stay on the chord for several bars, because a static minor chord grows tiring faster than a static major one.
Where it shows up
As the vi of C major and the ii of G major, plus tonic of its own key. In practice this makes A minor the most frequently played minor chord in guitar music after E minor.
The descending line Am–G–F–E — tonic down to the dominant through the notes in between — is old enough to have a name in classical theory and common enough to appear in rock, flamenco and film scores alike.