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Woodworking Math

Mortise and tenon joint

A mortise and tenon is, at its simplest, a peg in a matching hole: a tenon cut on the end of one piece fits a mortise cut into the side of the other. Rockler counts it among the three basic families of furniture joints, with butt joints and dovetails, and it is the usual way to join a rail to a leg or a stile in solid wood.

Why it holds

The USDA Wood Handbook says a plain end-grain butt joint cannot be glued strong enough for ordinary service, and that where end grain meets edge grain the joint needs interlocking parts, so that edge grain is glued to edge grain. It names dowels, mortise and tenons and rabbets as examples of strong joints of that kind.

One source: USDA Forest Products Laboratory, the authority for wood adhesive joints it tested

Proportions

A tenon is traditionally about one third the thickness of the stock being mortised.

The tenon’s thickness is usually set from the stock it goes into, so that the walls of the mortise on each side keep enough wood. For 3/4 in stock that rule gives a 1/4 in tenon, which is also a common mortise chisel size. Tenon length and mortise depth have no rule that the references agree on, so this page doesn’t give one; the mortise is cut a little deeper than the tenon is long so the shoulders close.

Traditional rule, stated by Popular Woodworking, Fine Woodworking (Taunton Press) and Woodcraft

Variations

A through tenon runs right through the mortised piece and shows on the far side; a blind or stopped tenon ends inside it. A haunched tenon keeps a short stub of the full width at the top of a frame, to fill the groove for a panel. A loose tenon is a separate piece glued into mortises in both parts, which is the system Festool’s Domino and router mortising jigs use.

How it’s cut

Mortises are cut with a hollow-chisel mortiser, a plunge router with a guide, or by hand with a mortise chisel after drilling out most of the waste. Tenons are cut on the table saw with a tenoning jig or a dado stack, on a band saw, with a router, or by hand with a tenon saw and a shoulder plane. Cut the mortise first, then fit the tenon to it.

Related joints

Sources

  1. Rockler, Woodworking furniture joinery: butt joints, mortise and tenon, dovetails, read 2026-11-07
  2. USDA Forest Products Laboratory, Wood Handbook (FPL-GTR-282, 2021), ch. 10 "Wood Adhesives", End-Grain and End-to-Edge-Grain Joints, read 2026-11-07
  3. Popular Woodworking, Christopher Schwarz, "Tenons Rule! So Here Are the Rules on Tenons" (2006), read 2026-11-07
  4. Fine Woodworking (Taunton Press), Gary Rogowski, "Tenoning Strategies", Fine Woodworking #142 (2000), read 2026-11-07
  5. Woodcraft, John English, "The Much-Heralded Mortise and Tenon", read 2026-11-07

Proportions marked as a traditional rule are shop tradition, not measurements. This site gives no joint-strength numbers: the published tests each used a different rig. How we source these numbers.