The open A shape is one of the first chords most guitarists learn, and one of the most flexible: nearly every common variation is a single finger moving from the base triad. This post works through those variations one finger-move at a time.

Base shape: A major

A major is the 1-3-5 triad: root, major third, perfect fifth.

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.

From the base A shape: sus2 lifts the middle finger off the third; sus4 shifts it up one fret. Both resolve naturally back to A major.

Minor and minor seventh

Am7 is Am with the note on the D string lifted -- one less finger than the major shape, not more.

Dominant seventh and major seventh

A7 pulls the third's neighbour down to add the flat seventh -- the chord used for the V in a blues in D (see the 12-bar blues post). Amaj7 instead adds a half-step below the octave, a jazzier, unresolved colour.

Sixth and add9

A6 barres the top three strings at the second fret to add the sixth degree without removing anything. Aadd9 stacks a ninth above the third for a bright, open colour without the suspended chord's lack of a third.

Summary table

Chord	Frets (low to high)	Change from A major
A	x02220	base shape
Asus2	x02200	remove 3rd
Asus4	x02230	3rd -> 4th
Am	x02210	major 3rd -> minor 3rd
Am7	x02010	Am, drop a finger
A7	x02020	add b7
Amaj7	x02120	add maj7
A6	x02222	add 6th
Aadd9	x02420	add 9th

Next in the series: the same treatment for the D-shape and E-shape open chords.