piano chord
A11
A1C♯3E5G♭7B9D11
Six notes in theory; in practice the third is usually dropped, because the eleventh sits a half step above it.
What is true of it
A11 is A, C♯, E, G, B and D — 1, 3, 5, b7, 9 and 11 counted from A.
On an instrument that cannot reach all 6, the 3, 5 and 1 go first, in that order.