Thinking out loud here, so to speak...

Suppose I have a song in G, and the only chords it contains are G, C, and D. One way to improv on it would be to play around in the G major scale in those bars where the chord is G, play around in the C major scale when the chord is C, and use the D major scale when the chord is D.

Or, if I take the G major scale and leave out the C because it would contradict the C# in the D scale, and leave out the F# because it would contradict the F in the C scale, what I'm left with is G A B D E G - a pentatonic scale. So presumably I can play around in this scale during the whole song with contradicting any of the chords?

Is that the idea of using a pentatonic scale for improvising - the notes it contains are the ones that are common to the song's basic chords?

If so, then maybe a major pentatonic scale is in order for a I-IV-V song, and other kinds of pentatonic scales are predictably associated with other predictable chord patterns?

Or have I got this all wrong - did you mean that I'm supposed to play in G pentatonic during the G chord bars, in C pentatonic during the C chord bars etc.?

Thanks, Marion