I was looking at an old Colour and Design work-book of mine from the mid ’80s and found this page on Tonal Scale and Value Keys. Still good theory and not a bad little painting exercise with gouache. I think it may have come from a Maitland Graves colour and design book. A couple of the hand-written notes read: Major because it has both ends of the scale involved, Minor because it has one end of the scale involved.
The exercise comes from these Maitland Graves books.