More in Home Improvement
The joint actually has two mortises. One board has a small mortise cut all the way through it. This board also has a tenon cut into it, which fits into the mortise in the center of the second board (Image 1). The first mortise ends up being recessed slightly into the larger mortise. The wedge, which is tapered, slides into the smaller mortise (Image 2). Driving the wedge into the mortise holds the piece together.
Use a mortiser to cut the small mortise first. Drill all the way through the board. Don’t cut the shoulders for the tenon first -- if this is done there will be no support for the board when attempting to cut the mortise later.
Use a dado stack on a table saw to cut the shoulders for the tenon. Start in the middle of the shoulder and work outward. Keep making passes until the shoulder is complete. Then cut the other side of the shoulder.
Return to the mortiser. In a second board, cut a mortise large enough for the tenon to fit through.