piano chord
Gadd9
G1B3D5A9
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
Gadd9 is G, B, D and A — 1, 3, 5 and 9 counted from G.
On an instrument that cannot reach all 4, the 5 goes first, in that order.