Paul Sellers demonstrates cutting a mortise joint by hand. He is using a method of showing the inside view of cutting the joint previously shown by Peter Follansbee who attributes it to Roy Underhill. This is to cut the mortise up against a piece of plate glass.
In this demonstration Paul Sellers shows cutting the mortise first with a mortise chisel and then with a bevel edge chisel.
via YouTube https://www.youtube.com/watch?v=q_NXq7_TILA