piano chord
A6
A1C♯3E5F♯6
A major triad with a sixth added. The same four notes as a minor seventh chord a third below.
What is true of it
A6 is A, C♯, E and F♯ — 1, 3, 5 and 6 counted from A.
These are also the notes of F♯m7/A and G♭m7/A. Which name is right depends on what is underneath and what came before — the notes alone do not decide.
On an instrument that cannot reach all 4, the 5 goes first, in that order.