piano chord
Fadd9
F1A3C5G9
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
Fadd9 is F, A, C and G — 1, 3, 5 and 9 counted from F.
On an instrument that cannot reach all 4, the 5 goes first, in that order.