I agree w you Roger. I learned it that way from the booklet that came w my 1st Stanley Steel Square. Then use slope (rise over run) analogously on the tongue and blade of the framing (Steel) square to step out the rafter Length (hypotenuese).
I see lots of old houses and barns w 8/12 roofs, some 9/12 too. But 8/12 is a noticeable break point, or break angle, for the human feet and balance not to slip or tumble easily. Working on a 9/12 slope roof demands much more attention and energy for balance and grip, and thus slows down the pace of the work significantly. "Time is money" also applied in the good old thrifty days. Gumshoes are good. 8/12 sheds rain and melting snow, taking along accumulated debris quite well. Lower pitch roofs hold more seeds, leaves, needles and dampness, giving moss, lichen, bacteria and fungus a good growing medium which can destroy roofing material faster than UV radiation. Finding balance - that's a good life. Seems the term pitch isn't used so much anymore. Steve