There are two wonderful dovetail joints on tie beams between the back wall and the swept beams. Along with two other horizontal ties they hold the back wall against the spreading/downward load of the roof. These joints will be self-locking on a green oak frame if the tie beam itself is cut from seasoned oak. You can see me here cutting a quarter way into the swept beam. It is a very satisfying joint to cut and this one was so snug that the two halves had to be wound together using a sash cramp. I cut these joints with a normal panel saw and a mortise chisel.