I understand the fretboard in terms of notes, meaning I can tell you what note you are playing if you're on the 8th fret on the g string. The only problem is I don't understand which scales to apply to a song that's in, say the key of C. Does that mean that the solo you play is always in C? Or does it mean you should start with C and end with C. I'm confused.

Let's use an example, say I play a chord progression G, D, E, what would you solo over? Would you start with playing in G, then switch to D, and end playing on E,or just solo in the key of G the whole time?

Hope everyone understands that.