piano chord
A♯m9
A♯1C♯♭3E♯5G♯♭7B♯9
A minor seventh with a ninth on top.
Written the other way round it is B♭m9 — the same twelve keys on a piano, a different piece of notation.
What is true of it
A♯m9 is A♯, C♯, E♯, G♯ and B♯ — 1, b3, 5, b7 and 9 counted from A♯.
On an instrument that cannot reach all 5, the 5 and 1 go first, in that order.