Guitar chords in the key of A:
A Bm C#m D E F#m Abdim

Chord A7 notes: A, E, G, C# and E. You should not play the 6th string.

The strings 5(A), 3(G) and 1(E) are left open.

A dominant 7th chord's alternative names: Adom7, (A septim).

Steps: 1-3-5-b7.
1(A), 3(C#/Db), 5(E), b7(G).