The parts of the joint
A mortise and tenon joint has two halves. The mortise is a rectangular pocket or slot cut into one piece, usually in the face or edge of a leg or stile. The tenon is the tongue left on the end of the other piece, usually a rail, which fits into that pocket.
A few terms help when reading plans:
- Cheeks: the broad faces of the tenon, which glue to the walls of the mortise.
- Shoulders: the flat surfaces where the tenon meets the rest of the rail. They sit against the face of the leg and stop the rail from twisting or racking.
- Haunch: a short extra step left at the edge of a tenon, filling the end of a groove or keeping the rail from twisting.
- Through tenon: goes right through the mortise and shows on the far side. A blind tenon stops inside.
Why it holds
The cheeks give large long-grain gluing surface, which is the strongest kind of glue bond in wood. The shoulders bear against the leg and resist the rail from twisting out of square. Together, the joint resists racking better than a simple butt joint, which is why it is used in table frames, doors, chairs, and bed rails.
A common rule of thumb is to make the tenon about one third of the thickness of the rail, so the mortise does not weaken the leg too much. The tenon length is often chosen to go roughly two thirds or more of the way through the leg for blind joints. Treat these as starting points rather than strict rules; your project and the wood species change what is sensible.
Layout first
Accurate layout makes the joint easier. Mark both parts at the same time, with the pieces placed together, so the positions match. A marking gauge with two pins or two settings can scribe both walls of the mortise and both cheeks of the tenon from the same reference face. Using the same face on every piece keeps parts aligned even if the stock varies slightly in thickness. Mark shoulder lines with a marking knife, and mark waste with pencil.
Cutting the mortise
Common methods include:
- Chisel and mallet. Chop out the waste in stages, working down in small steps and levering waste out. Bench chisels and a heavier mortise chisel are used. The ends are chopped, then the walls pared straight.
- Drill and chisel. Drill a row of overlapping holes with a bit slightly smaller than the mortise width, preferably in a drill press so holes stay square, and clean up the walls with a chisel. This is the approach many beginners find friendly. Drill depth stops help.
- Hollow chisel mortiser. A dedicated attachment or machine presses a square chisel around a rotating bit. It cuts square holes fast, but takes setup. Follow the manual closely.
- Router. A plunge router with a straight bit and a guide cuts mortises with rounded ends. The tenon corners are then rounded to match, or the mortise ends are squared with a chisel.
Cutting the tenon
- Hand saw. Cut the cheeks along the grain down to the shoulder line, then cut across the shoulders. Saw on the waste side of the line and pare to it.
- Table saw. The tenon can be cut using a miter gauge, a tenoning jig, or a series of passes with a standard or dado blade. This is repeatable but needs care, with the workpiece held securely and fingers kept away from the blade. A jig that holds the piece upright must be used according to the manufacturer’s directions.
- Router table. With a straight bit and a miter gauge or sled, you can trim the cheeks and shoulders in shallow passes.
- Bandsaw. A bandsaw can cut cheeks quickly, followed by cleanup with a shoulder plane or chisel.
Fitting and assembly
The tenon should slide in with firm hand pressure, without needing a hammer. Test fit often. If the fit is tight, shave the cheeks very lightly with a chisel or shoulder plane. If the fit is loose, a thin shim is preferable to excess glue.
Dry fit the entire frame before gluing, then check that it is square by measuring diagonals. Glue the long-grain surfaces of the tenon and mortise walls; avoid putting glue at the bottom of a blind mortise, where it can trap air or hydraulically resist the tenon. Cutting a small relief at the bottom, or a tiny groove along the tenon, gives glue and air somewhere to go. Clamp across the shoulders and check the assembly for square.
Pinning a joint with a dowel through the cheeks (drawbored pins are an old, strong method) can add extra security, though the details deserve their own study.
Safety
Wear safety glasses, hearing protection with powered tools, and a dust mask when sanding or machining. Clamp the work when chopping. Keep your hands behind the chisel’s edge, unplug power tools before changing blades or bits, and follow the manufacturer’s manual. This is general information, not professional advice.