piano chord
C6
C1E3G5A6
A major triad with a sixth added. The same four notes as a minor seventh chord a third below.
What is true of it
C6 is C, E, G and A — 1, 3, 5 and 6 counted from C.
These are also the notes of Am7/C. 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.