Mortise-and-Tenon Joint Layout and Cutting
The mortise-and-tenon joint is arguably the most important woodworking joint in history — it has connected furniture, timber frames, and architectural elements for at least 7,000 years. Its strength derives from the interlocking mechanical geometry: a tenon (a tongue of wood projecting from the end of one piece) fits precisely into a mortise (a rectangular cavity cut into the mating piece). The proportions of a well-designed mortise-and-tenon are not arbitrary: the tenon thickness should be approximately one-third the thickness of the stock (a 3/4" thick rail gets a 1/4" tenon); the tenon width should leave equal shoulders on all four sides (shoulders provide resistance to racking force — a tenon without shoulders has no racking resistance). A through tenon passes completely through the mortised piece and is visible on the far side — used structurally in timber framing and aesthetically in exposed furniture. A blind (stopped) tenon stops inside the mortised piece and is invisible when assembled. Layout begins with the mortise gauge: set the gauge's two pins to exactly the tenon thickness (and therefore the mortise width) and score both the mortise and tenon layout lines simultaneously, ensuring perfect correspondence. Mark the mortise length based on the tenon width. Cutting the mortise: remove the bulk of the waste with a series of 1/8" drill bores (staying inside the layout lines) then pare the walls precisely to the scored lines with a sharp bench chisel held vertically. The mortise walls must be flat and perpendicular — any taper causes a loose fit on one end and interference on the other. Cutting the tenon with a tenon saw: saw the cheeks (the long faces) first, staying just outside the layout line on the waste side. The shoulder cut (across the grain, square to the tenon face) is then cut with a crosscut saw in a bench hook. Final fitting: the tenon should slide into the mortise with hand pressure — no force required, but no side-to-side wobble.