I am learning After Midnight (Jj Cale). A really nice song, not compicated, simple, but very rich. Kinda of like Good Riddance. Brilliant in its simplicity. My opinion anyway 
The chords in AM are D F G A. Am I correct to say it is in the key of D even though it uses F major instead of F minor?
Thanks in advance for your response.
I think it is just three basic chords:
D, F, G
Try capo fret 5 play A-shape, C-shape and D-shape respectively.
Key of D
D major scale:
D, E, F#, G, A, B, C#
1, 2, 3, 4, 5, 6, 7
Diatonic chords
D, Em, F#m, G, A, Bm, C#dim
I, ii, iii, IV, V, vi, vii
The F major chord is non-diatonic. Its root note is the b3 of the scale. It is major so is named bIII. It is a borrowed chord.
| D | F G | D |
| I | bIII IV | I |
Sorry about the AM (After Midnight not A minor) in my first post. Will be careful in the future to avoid confusion.
I am still relatively new to theory so the exceptions to the rules are vexing.
If I am understanding correctly, the bIII is a “borrowed chord”. Even though F is not in the key of D (F# is the actual 3rd in that key) like Hey Joe (C G D A E) it is ok because it sounds good regardless of the rules. It does not follow the major and minor chords of a key but like Jimi said, if it sounds good, it sounds good (paraphrasing).
Is this correct? Mostly correct?
Attila
Correct!