The open A shape is one of the first chords most guitarists learn, and one of the most flexible: almost every common variation is a single finger moving from the base triad. Each diagram below shows the fingering on the left and the intervals that fingering spells on the right.
Base shape: A major
A major is the 1-3-5 triad: root, major third, perfect fifth. Four notes sound across five strings, so two of them are doubled. The B string carries the third, and the G string carries a doubled root. Those two strings are where every variation below happens.
Suspended: sus2 and sus4
Suspended chords replace the third with the second or fourth -- no major or minor quality, just tension resolving back to the triad. Both moves happen on the B string. Lift the ring finger and the open B gives the second; slide that same finger up one fret and D gives the fourth.
Minor and minor seventh
Am takes the B string third down one fret to C, the minor third. Am7 then releases the doubled root on the G string to the open G, adding the flat seventh -- one finger fewer than the major shape, not more.
Dominant seventh and major seventh
A7 makes the same move on the G string but against the major third: lift the middle finger, let the G ring open, and the doubled root becomes the flat seventh. The rest of the A major hand does not move. It is the V chord in a blues in D (see the 12-bar blues post). Amaj7 takes that string down only one fret instead, to G#, a half step below the root, for a jazzier and unresolved colour.
Sixth and add9
A6 is the one exception to the single-finger rule. Barre the second fret across the D, G, B and top E strings, and the open high E, a fifth, becomes F#, the sixth. Aadd9 goes back to the G string and stretches it up to the fourth fret, turning the doubled root into B, the ninth. Both keep the third, which is what separates an add chord from a suspension.
Summary table
| Chord | Frets (low to high) | Change from A major |
|---|---|---|
| A | x02220 | base shape |
| Asus2 | x02200 | B string: 3rd -> 2nd (open) |
| Asus4 | x02230 | B string: 3rd -> 4th |
| Am | x02210 | B string: 3rd -> b3 |
| Am7 | x02010 | Am, plus G string: doubled root -> b7 (open) |
| A7 | x02020 | G string: doubled root -> b7 (open) |
| Amaj7 | x02120 | G string: doubled root -> maj7 |
| A6 | x02222 | top E: 5th -> 6th, barre at fret 2 |
| Aadd9 | x02420 | G string: doubled root -> 9th |
Next in the series: the same treatment for the D-shape and E-shape open chords.