The Fiddle Place

If a song is in the key of A then the One Chord will be A, The song will usually have three.
They are the I , IV and the V.  Counting from 1, A is the I chord,  D is the IV chord and E is the V chord.  I also show the layout for the key of G.

The 1,3 nad 5 notes are "legal" to play for that chord.

1

3

5

I

A

A

C#

E

IV

D

D

F#

A

V

E

E

G

B

Key of G

I

G

G

B

D

IV

C

C

E

G

V

D

D

F#

A