To really outline the chords in this study I am drawing on each chords related mixolydian scale. G7 takes G mixolydian G A B C D E F G. When we switch to C7 ( C E G Bb) we must change our scale to accommodate the chord. That scale would be C mixolydian C D E F G A Bb C. Whoa, that's only one note different! Yup, it's that simple. But as we know simple isn't always easy... The V7 chord is D7 (D F# A C) logically we need to take care of the F# properly. That would give us D E F# G A B Cor D mixolydian. SO once again we need only change one note from out parent scale to cover the changes. Pretty cool!