piano chord
B♭maj9
B♭1D3F5A7C9
A major seventh with a ninth on top.
Written the other way round it is A♯maj9 — the same twelve keys on a piano, a different piece of notation.
What is true of it
B♭maj9 is B♭, D, F, A and C — 1, 3, 5, 7 and 9 counted from B♭.
On an instrument that cannot reach all 5, the 5 and 1 go first, in that order.