Originally Posted by: Jolly McJollyson???

If it's in the key of C you'd play a Cmajor scale. If it's in the key of Aminor you'd play an Aminor scale. That's what key is. It's based off the tonic scale.

If the song is in the key of "C" you can play other scales than the obvious.
for example you can play:
A minor Pentatonic. [A,C,D,E,G]
D minor Pentatonic. [D,F,G,A,C]
G Pentatonic......... [G,A,B,C,D]
The reason you can play these scales is because they contain the notes of the "C major scale"