What nominal lumber size is 11/16?
"1 inch" with an additional trip through the planer - ie, it's not stock. For that matter, what is "stock" for various sizes varies with vendor, over time, etc. - the place I used to get hardwood in central NY planed 4/4 to 13/16" unless you asked for a different thickness, so 3/4 would not be "stock" there.

With rare exceptions any lumber thinner than 1 inch is made by planing from 1 inch (actually "4/4 rough") in practice. A well equipped lumberyard (not home cheapo) can custom-plane it for a nominal fee, otherwise you are going to need access to a planer to get this size.