piano chord
C♯6
C♯1E♯3G♯5A♯6
A major triad with a sixth added. The same four notes as a minor seventh chord a third below.
Written the other way round it is D♭6 — the same twelve keys on a piano, a different piece of notation.
What is true of it
C♯6 is C♯, E♯, G♯ and A♯ — 1, 3, 5 and 6 counted from C♯.
These are also the notes of A♯m7/C♯ and B♭m7/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.