piano chord
Cadd9
C1E3G5D9
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
Cadd9 is C, E, G and D — 1, 3, 5 and 9 counted from C.
On an instrument that cannot reach all 4, the 5 goes first, in that order.