Uhhh, I received many cards and letters asking that I not use a 7-string to demonstrate to a mostly 6-string crowd, especially not one with no fret markers.
I still have one 6 string electric, so I used it this time.
Yeah, it's hard to capture all the nuances of the chord forms in words.
The D and C chord shapes are basically the same.
The A and G chord shapes are basically the same.
The scales assosiated with D/C, and/or A/G... I think of those as being one position apart.
The D scale being behind the D chord form.
The C scale being above the D chord form (although now I'm thinking C shape).
For example, just before the solo where I say, A, E, A (the actual chords are A, E, F#), I probably should have called it A shape, E shape, G shape.
I think the difference is if you are covering those 3 strings with your ring finger (A-shape), or index finger (G-shape).