piano chord
D6
D1F♯3A5B6
A major triad with a sixth added. The same four notes as a minor seventh chord a third below.
What is true of it
D6 is D, F♯, A and B — 1, 3, 5 and 6 counted from D.
These are also the notes of Bm7/D. 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.