The slip tenon connects the top of the arch boards together. It remains loose until all the joints of the top are dry fitted together exactly. The grain is oriented horizontally for maximum strength. These mortise and tenon joynts on the door stiles and the very top of the arch are the only glued parts of the whole door. No joynts of raised panel walls, wainscoting, windows or interior and exterior doors were glued, wood swells and shrinks a lot ( except in the construction of interior work as the top of arches as this one) in the 18th century.. Hyde glue is heat and water soluble - whats the point for exterior application.