piano chord
Dadd9
D1F♯3A5E9
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
Dadd9 is D, F♯, A and E — 1, 3, 5 and 9 counted from D.
On an instrument that cannot reach all 4, the 5 goes first, in that order.