Mortise and Tenon Joint: Strength, Types and Layout

A mortise and tenon joins a rail to a post by cutting a tongue on the end of the rail and a matching socket in the face of the post. It is the default frame joint because it answers the grain problem head on: instead of gluing the rail's end grain to the post's face, the tenon cheeks present long grain to the long grain of the mortise walls, which is the bond wood glue actually holds.

The Wood Handbook lists mortise and tenon among the strong end-to-edge-grain connections precisely for that reason, alongside dowels and rabbets, and states the underlying rule: design the joint with interlocking surfaces so that edge grain bonds to edge grain, and increase the joint surface area to provide more bondline to transfer load.

Published tests keep the claim honest. Matthias Wandel of Woodgears clamped rails into posts and loaded them 20 cm out until they broke. Rails were 6 x 1.7 cm, posts 3.5 x 3.5 cm, tenons 10 mm thick, 27 mm deep and 55 mm wide, glued with yellow carpenter's glue.

In spruce the mortise and tenon joints failed at 170, 175, 140, 180, 185 and 190 lb, averaging 172 lb, against dowel joints at 120, 140 and 145 lb, averaging 135 lb. In maple the same joint reached 300 and 270 lb, dowels 230 and 245 lb. So the mortise and tenon wins, but by about 25%, not by the margin the joint's reputation suggests. Splitting the joined pieces afterwards showed post fibres stuck to the tenons, meaning the wood failed rather than the glue.

The other finding is more useful in the shop: tenons ranging from hammer-tight to a few thousandths loose showed no detectable difference in strength, as long as glue was present everywhere in the joint.

Egyptian carpenters had the joint by the 1st dynasty and had already branched it out. A published survey of their techniques separates the main forms.

• Stub mortise and tenon - the mortise does not go through and the tenon is shorter than the workpiece, so nothing shows. Used for stools and simple frames, sometimes secured with a dowel through the face into the tenon cheek.
• Through mortise and tenon - the tenon passes right through and is visible on the far side. Used in bed frames as early as the Old Kingdom; the survey cites a frame from Saqqara, inventory number 480.
• Locked or pegged mortise and tenon with a loose tenon - two mortises cut in the edges of two planks with a separate rectangular tenon inserted into both, then locked by driving a dowel through holes in the mortise wall and tenon. Used extensively in boatbuilding, statuary and coffins.
• Bridle joint - the survey describes it as similar, except that the mortise and tenon are cut to the full width of the tenon member.

Modern loose-tenon joinery is the same idea as the third form: a separate tenon in two mortises.

Proportions first. A tenon about one third the thickness of the rail is the usual starting point, because it leaves mortise walls strong enough not to blow out and a tenon thick enough not to snap. Depth matters more than most beginners expect - the test tenons above were 27 mm deep into a 35 mm post - and width does the most work of all, since the glue area grows directly with it.

Then the sequence. Cut the mortise first and fit the tenon to it, because a mortise is hard to adjust and a tenon is easy to pare. Reference every measurement from the show face of both parts so any error lands on the inside. Leave shoulders square and full-contact; the shoulder, not the cheek, is what stops the rail rotating.

Glue both the mortise and the tenon. The Wood Handbook notes that end-grain surfaces are many times more porous than radial or tangential surfaces and that adhesive overpenetration leaves voids in the glue line, so a thin single-sided smear is the wrong economy here.

Use dowels when speed matters more than the last 25% of strength and you have a jig that indexes accurately; the published gap between the two joints is real but small. Use a pocket-hole screw for face frames, jigs and shop furniture, accepting that the same tests found screwed joints deflect badly before they ultimately fail. Use a lap joint where the two members can overlap instead of meeting end to face, since that turns the whole connection into long grain against long grain.

And remember the handbook's warning about this entire family: because wood swells much more across the grain than along it, moisture changes produce large internal stresses in end-to-edge-grain joints, so they should be protected from changes in moisture content in service. A chair that lives next to a radiator is fighting its own joints.