However, AFAIK the true origin of the lapel buttonhole, at least on double-breasted jackets, is that the jackets from which the DB was adapted did actually button all the way up when the lapel was folded over all the way. If you have a good peacoat, there should be buttons usually hidden by the collar that are for buttoning the top of the lapel to in very cold or windy weather.
This is correct. Â The modern SB coat is derived from the ghillie collar coat, a sporting coat worn mostly in the country. Â The ghillie collar looks sort of like a shirt collar. Â The coat buttoned at the neck, right under the center of the collar. Â Men increasingly left the top button unbuttoned, and wore neckties with the coat. Â It wasn't long before they simply folded the upper coat edges back. Â The gap between the ghillie collar and the top of the coat front became the notch in a notched lapel. Â Hardy Amies' book The Englishman's Suit
has photo that show this evolution quite clearly. Anyway, the buttonhole is a relic of when the lapel didn't fold back over the front of the coat, and wasn't really a lapel at all, but was simply the front of coat that buttoned all the way to the throat. The DB story is similar, but a little different. Â The modern DB coat is derived from the frock coat, which derived as a sort of hybrid from the riding coat and the military tunic. Â These coats had a front flap that overlapped accross the chest. Â The military tunic buttoned all the way up. Â Lapels are derived from folding back the front flaps. Â The buttonhole remains from the days when the coat closed all the way up.