Understanding Mortise and Tenon Joints for Strong Frames

Why this joint still earns its place in modern homes

If you have ever admired a solid oak door that swings without a creak, or a window frame that has survived decades of British weather, you have likely seen a mortise and tenon joint at work. This ancient pairing of a rectangular hole (the mortise) and a matching peg (the tenon) remains the gold standard for strong frames. It resists twisting, racking, and the constant push and pull of daily use. For doors, windows, and furniture, it offers a mechanical strength that screws and biscuits simply cannot match. As a carpenter and joiner, I reach for it whenever a project needs to last.

Getting the proportions right before you cut

Good joints begin with good maths. Before you sharpen a chisel, decide the size of your mortise and tenon. A reliable rule of thumb is that the tenon thickness should be one third of the thickness of the timber being mortised. If your stile is 60mm thick, aim for a 20mm tenon. The mortise width should match your chisel exactly, so you are not fighting a sloppy fit. For length, a tenon should run between two thirds and three quarters of the width of the piece it enters. A 100mm wide rail, for example, takes a tenon around 70mm long. For exterior doors and windows, add a haunch — a small stub of tenon above the main shoulder — to stop the rail twisting.

• Tenon thickness: one third of the mortised timber's thickness.
• Mortise width: identical to your chisel width (common sizes: 6mm, 8mm, 10mm, 12mm).
• Tenon length: two thirds to three quarters of the mortised piece's width.
• Haunch: essential for doors and windows to prevent twisting.

Marking out: the difference between a snug fit and a sloppy one

Accurate marking saves hours of fiddling. Start by choosing a reference face and edge on both pieces — usually the face you want showing. Use a mortise gauge set to your chisel width to mark the mortise on both faces of the stile. Press the gauge fence firmly against the reference edge, and score a knife line rather than a pencil line. The knife cuts fibres cleanly, so your chisel sits in a tiny groove. For the tenon, set the same gauge to the same width and mark the cheeks. Then use a square and knife to mark the shoulders all the way around. Never assume the timber is square; check with a try square and adjust your lines accordingly.

Cutting the mortise without tear-out

Boring out the waste first makes chiselling far easier. Use an auger bit or Forstner bit slightly narrower than the mortise, and drill a series of overlapping holes down the centre. Keep the drill perpendicular to the face. Then chop the remaining waste with a mortise chisel. Stand the chisel upright, bevel facing the centre of the mortise, and strike it with a mallet. Work from both ends towards the middle to avoid splitting the ends. After the first pass, pare back to your gauge lines with a bevel-edged chisel. Check the walls are square and flat by eye and with a small square. A clean mortise has crisp corners and no bruising on the surrounding timber.

Cutting the tenon: shoulders, cheeks, and haunch

Now for the tenon. Clamp the rail securely, then saw down the cheeks on the waste side of your knife lines. Use a tenon saw with fine teeth, and let the saw do the work. Stop when you reach the shoulder line. Next, cut the shoulders. This is where accuracy matters most, because the shoulder transfers the load. Saw just to the line, then chisel back to it for a perfect 90-degree corner. If you are adding a haunch, cut it after the main shoulders. Test the fit with a dry assembly. The tenon should slide in with firm hand pressure, not a hammer. If it is too tight, pare the cheeks with a shoulder plane or chisel. If it is loose, you need a new piece — glue and sawdust will not rescue a sloppy joint.

Fitting, gluing, and draw-boring for a lasting hold

Once the joint fits, apply a good-quality woodworking glue to both the mortise walls and the tenon cheeks. Assemble promptly, because most glues grab quickly. Clamp diagonally across the frame to pull the shoulders tight. For the strongest hold, consider draw-boring. Drill a hole through the mortise and tenon, but offset the hole in the tenon by 1–2mm towards the shoulder. When you drive a slightly tapered oak peg through, it pulls the joint together with immense force. This traditional technique is perfect for doors and windows that face the elements. Wipe off excess glue with a damp cloth, and leave the frame to cure flat and square. With care, your mortise and tenon frame will outlast the building it sits in.