THIS!!!
I've seen a lot of people struggle over the years, trying to wrap their heads around music theory, just by remembering, what appears to be, just a bunch of formulas, that are all completely correct, but have no point of practical reference to ANYTHING useful for making music. Once you show them that they can grab the instrument and actually play chords in an ascending order, it seems like EVERYTHING becomes clear... In fact, in Chuck's Major key example, if you complete the scale by adding the A-dim, you will, (probably for the first time), HEAR how the A-dim resolves back to the C chord, and THAT is the beginning of understanding the
"CONTEXT" that he is referring to...
Try this... Using the F major scale, play the first three bar chords,
F - Gm - Am with a sixth string root, and then play the remaining chords,
Bb - C - Dm - Edim with a fifth string root, and then end by going back to the
F... You now have a sonic reference for EVERYTHING in music theory...
NEXT STOP???
Arpeggios...