How to Cut a Mortise and Tenon Joint

A mortise and tenon is a peg cut on the end of one part fitted into a hole in the other. It is the strongest joint in furniture because the glue works on long grain against long grain, and because the shoulders resist racking mechanically rather than relying on adhesive. Neither pocket screws nor dowels match it, which is why chairs are made this way.

The proportions

Two rules cover almost every case, and both exist for a mechanical reason.

The one-third rule

Make the tenon about one third the thickness of the stock. In 3/4-inch material that is a 1/4-inch tenon, with 1/4 inch of cheek on each side. This is not arbitrary: too thin and the tenon shears; too thick and the mortise walls become fragile and split. A third splits the difference, and 1/4 inch conveniently matches a common chisel and router bit width.

Depth and length

A tenon roughly two-thirds to three-quarters into the width of the receiving part is typical. Do not bottom the tenon out — cut the mortise about 1/8 inch deeper than the tenon is long, so glue and air have somewhere to go and the shoulders can pull tight against the face.

Shoulders

The shoulders are what you see, and they are what makes the joint look good or bad. A tenon can be sloppy inside the mortise and the joint will still look perfect if the shoulders seat cleanly against the face. That single fact should shape how carefully you cut each part.

Cut the mortise first. Always.

This is the one rule that saves the most joints, and beginners routinely get it backwards. It is far easier to trim a tenon down to fit a mortise than to widen a mortise to fit an oversized tenon — you can pare a tenon cheek with a shoulder plane or a chisel in seconds, and widening a mortise accurately is genuinely difficult. Cut the hole, then fit the peg to it.

Cutting the mortise

By hand

Mark the mortise with a squareand a marking gauge, then chop it with a chisel the exact width of the mortise — the chisel becomes the width gauge, which is why the one-third rule's convenient 1/4 inch matters. Work from the middle outward, levering waste out as you go, and stop short of your end lines until the bulk is removed. Pare to the lines last. A sharp chisel makes this pleasant; a dull one makes it miserable.

With a drill and chisel

The practical hybrid most people use. Drill a row of overlapping holes with a Forstner bit slightly under the mortise width, using a drill press or a guide to keep them vertical, then clean the walls square with a chisel. Faster than chopping and more accurate than most people expect.

With a router

A plunge router with an edge guide cuts a clean, consistent mortise fast. The mortise will have rounded ends, which you either square with a chisel or match by rounding the tenon corners — both are legitimate, and rounding the tenon is quicker.

Cutting the tenon

By hand

Mark the shoulders all the way round with a knife, not a pencil — the knife line gives your saw a wall to register against. Saw the cheeks first with the workpiece angled in the vise, cutting on the waste side of the line, then saw the shoulders. A tenon saw is the tool for this; a dovetail saw is too small and too fine.

On a table saw

With a dado stack or repeated passes over a standard blade, using the mitre gauge and a stop block for the shoulder position. Fast and extremely repeatable, which matters when a table needs eight identical tenons. See our table saw roundup.

On a bandsaw

Cheeks on a bandsaw against a fence, shoulders elsewhere. Safer than a table saw for standing stock on end, and a good option for one-off joints.

Fitting

Aim for a tenon that slides in with firm hand pressure — snug, not hammered. A joint you have to mallet home will squeeze all the glue out and can split the mortise walls when the wood swells with moisture from the glue.

Trim the cheeks, not the mortise. A shoulder plane is the ideal tool because its blade reaches into the corner; a sharp chisel used as a paring tool works fine. Take a shaving at a time and re-test; it is very easy to go from tight to loose in one pass.

If the tenon is too loose

Glue a shaving or a thin slip of the same species to one cheek, let it dry, and re-fit. This is a standard repair, not an admission of failure — it is invisible once assembled and stronger than filling the gap with glue, which produces a weak joint that looks fine until it does not.

Glue-up

Dry-fit the whole assembly first, with clamps, and check it is square by measuring both diagonals. Then glue: coat the mortise walls and the tenon cheeks — the cheeks are where the long-grain glue surface is and where the strength comes from. Clamp across the joint so the shoulders pull tight, and leave it properly. A glue with a longer open time makes a multi-joint assembly far calmer; see best wood glue.

For a joint that must never fail — a chair — pegging or draw-boring adds a mechanical lock that survives even if the glue eventually lets go. That is why centuries-old chairs are still standing.

What we could not verify

We have not conducted destructive testing on mortise-and-tenon joints. The proportions above are the standard, widely published guidance for the joint, and the reasoning for them is mechanical: tenon thickness trades shear strength against mortise-wall strength. Treat the one-third rule as a well-established starting point, not a measured optimum.