beerfreak, It seems to me it would also depend upon lay-out method which would depend on the wood. It seems that if the square rule method was used and thus housings had to be cut because of different dimensioned timbers it would make more sense to have the shoulder run to an outside face. It takes a lot less time to cut a housing for a brace if you can cut from an edge as opposed to cutting a housing with wood on each side of it. I'm no expert on tradtional barns but it seems that with the tools available it would have been a certain mess to center a brace in square rule without a router, instead run the brace to a face, handsaw, then chisel.