The A minor ninth chord is a 5-note chord consisting of the notes A, C, E, G and B.
You can see these notes highlighted in the interactive piano chart below.
The chord itself is often abbreviated as Am9.
Chords that are a superset of Am9. The chords include more notes but always A, C, E, G and B.
Chords that are a subset of Am9. These contain less notes but all of them are included in Am9.
In case you prefer a non-interactive variant of the chord chart, we've embedded a PNG image below that shows the notes for the the Am9 chord. Feel free to save or share the image as needed.