mortise-and-tenon joint

A mortise-and-tenon joint joins two pieces of wood by carving a rectangular hole, called a mortise, into one piece, and shaping a projecting tongue, called a tenon, on the end of the other piece that fits into it. The tenon is glued and driven or seated into the mortise, creating a strong mechanical and adhesive bond. The joint can bear loads perpendicular to the grain without relying solely on glue strength, which is why it remains standard in furniture and structural work where wood movement and longevity matter.

The tenon is typically cut on the end grain of one component, narrower and shorter than the full thickness and width of the piece it leaves. The mortise is chiseled or routed to match, stopping just short of piercing the far face. A through mortise, where the hole goes all the way across, exposes the tenon end on the outside; a blind mortise hides it. The gap between tenon and mortise walls, often 0.5 to 2 mm depending on wood type and fit preference, allows glue to spread and accommodates seasonal wood movement without splitting.

Grain orientation matters. A tenon runs with the grain of its piece, giving it strength in compression along its length. The mortise is cut across the grain of its piece, which is weaker, so the mortise piece must be thick enough to resist crushing at the walls. Proportions vary by application: a typical rule is to make the tenon about one-third the thickness of the piece it leaves, though structural work may call for thicker tenons and deeper mortises.

Common failures and variations

Inadequate glue coverage, misalignment during assembly, and wood movement after the joint sets cause most failures. Seasonal humidity changes move wood along and across the grain at different rates; a mortise-and-tenon can resist this better than edge joints, but poor grain direction or finish choices still split shoulders and frames. Multiple smaller tenons, called split or haunched tenons, distribute the stress and allow more flexibility than a single large one. Wedged tenons, where a wooden wedge is driven into a saw kerf in the tenon after assembly, lock it mechanically and compensate for later shrinkage.

The joint name reflects its two components: the mortise derives from Old French for a fastening hole, and the tenon from Old French for a holding piece. It is older than machinery; woodworkers cut both by hand with chisels, handsaws, and gauges, and it remains a benchmark of hand skill. Modern workshops may rout mortises with precision jigs and cut tenons on tablesaws, but the geometry and assembly discipline have not changed. Chair stretchers, door frames, box lids, and structural timber beams all rely on this joint because it works reliably across centuries and climate zones.