piano chord
Aadd9
A1C♯3E5B9
A major triad with a ninth added and no seventh — which is what tells it apart from a ninth chord.
What is true of it
Aadd9 is A, C♯, E and B — 1, 3, 5 and 9 counted from A.
On an instrument that cannot reach all 4, the 5 goes first, in that order.