Sawing and chiseling the slip mortise pocket for the slip tenon
 
Sawing and chiseling the slip mortise pocket for the slip tenon

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 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.