A Major 6th
6
0 - 4 - 7 - 9Major + major 6th
About A Major 6th
A6 is A major with an F♯ added, and on guitar the addition is nearly free: the F♯ sits at the second fret of the high E string, right where the hand already is. The result is a bright, slightly nostalgic chord that country and swing players use where others would use a seventh.
The notes
A, C♯, E, F♯. The F♯ is a major sixth above the root, and it turns a plain A into something that sounds finished rather than expectant.
The same notes read from F♯ give F♯ minor seventh — the relative-minor equivalence again, and the reason A6 can substitute for F♯m7 in an arrangement without the harmony changing.
On the fretboard
The simplest shape is the standard A with one finger moved to the second fret of the high E. A fuller version frets all four notes on adjacent strings and mutes the rest, which is the swing rhythm voicing.
Because the sixth is available open on no string, every A6 shape frets it — but always within the second-fret block where the A major shape already sits.
Where it shows up
As a tonic in A major, chiefly at the end of phrases in country, western swing and early rock and roll. It also appears in the turnaround of a blues in A, where a plain triad would sound too plain.
In jazz it serves as a rest chord where a maj7 would be too smooth for the style — the sixth is drier, and that dryness is period-specific.
The chord also marks a boundary in guitar history: rock and roll inherited it from swing and then largely abandoned it for the seventh, so a recording that ends on A6 usually predates 1960 or is deliberately quoting something that does.