So I picked up a 1997 copy of this book: http://www.amazon.com/Country-Guitar-Music-Sales-America/dp/186074138X/ at the local used book place for a couple bucks. One bit has me hopping. The guy goes on about modes and other theory stuff, but messes up something pretty simple. He's got a bit on country using thirds and sixths, and not only doesn't bother to relate them, practically says that they don't relate. Well friends, they do relate. A sixth is nothing more than a going a third lower, and inverting the resulting interval. In A, C is the third. In C, A is the sixth. So from his perspective, sixths are just the thirds of the relative minor, inverted.