Adding to Glenn's post. With a non-synchro tranny shifting isn't real easy. Double clutching stops the gears and shafts from rotating at different speeds.
I've always hated double clutching. Learned how to shift non-synchro trannies in diesels without using a clutch. I apply the same methods to both synchros and non synchros. You match the engine rpm to the driveshaft rpm and slide it into gear.
