S&D, S&R, Trends, Fork's and Turnarounds, page-472

Time out on the edit, but wanted to add:

I should add - I've come to see them in the same way as median lines now, it took me years to feel confident about that, but it is just how they work.  Remarkable to me.  I suspect now, that they work in a similar way to Fibonacci spirals, like the ones you get on shells and stuff. Still pondering that one.