How do I cut a mortise and tenon joint by hand?

Marking from one face, cutting the mortise first, sawing the tenon in the waste, and paring to a fit rather than sawing to it.

Difficulty: intermediate
Time: 1 hr 30 min
Read: 4 min
Safety: caution

Short answer

Mark everything with a gauge from the same face of each piece, chop the mortise first, then saw the tenon slightly fat and pare it down until it fits. The mortise comes first because it is far easier to adjust a tenon than a hole.

The joint is not difficult so much as unforgiving of careless marking. Almost every problem with it traces back to gauging from different faces on the two components.

Safety

Chisels are pushed towards the work with real force. Keep both hands behind the cutting edge, clamp the workpiece rather than holding it, and never chop towards your body or over your other hand. A sharp chisel is safer than a blunt one because it needs less force and is far less likely to skid.

What you'll need

A mortise gauge or marking gauge
A mortise chisel or a firm bevel-edge chisel
A tenon saw
A mallet and clamps

Step by step

Mark a face side and face edge on both pieces.Pencil marks that you always gauge from. The single biggest cause of a joint that will not sit flush is measuring the mortise from one face and the tenon from the other.
Set the gauge once, for both parts.Set the mortise gauge to the chisel width, then mark both the mortise and the tenon cheeks without changing it. The two now match by definition rather than by measurement.
Aim for a tenon about a third of the thickness.Roughly one third the thickness of the rail is the traditional proportion. Matching the tenon to a chisel width you actually own is more practical than a precise fraction.
Mark the mortise length and knife the shoulder lines.A knife line rather than pencil, right round the rail for the tenon shoulders. The saw and the chisel both register against a knife line, which is what makes shoulders crisp.
Chop the mortise first.Working from the middle outwards, chisel bevel towards the waste, taking small bites and levering the waste out. Stay a millimetre inside the ends until the last cuts, then pare down to the lines.
Keep the chisel vertical.Sight from the side and the end, or clamp a square block beside the mortise as a visual reference. A leaning mortise is very hard to correct.
Saw the tenon cheeks in the waste.On the waste side of the gauge line, leaving the line visible. Saw the corner nearest you at an angle, then the far corner, then level out and cut down to the shoulder.
Saw the shoulders last.Into the knife line, with the workpiece held against a bench hook. A slightly undercut shoulder pulls the joint tight at the visible edge.
Pare to a fit.Try the joint. It should go together with firm hand pressure. Pare the cheeks with a wide chisel a shaving at a time — it is a fit you sneak up on, not one you cut in a single go.
Check for square while dry fitting.Assemble dry and check with a square in both planes. Correct now, because glue and clamps will not straighten a joint that is out.
Glue and clamp across the joint.Glue on the mortise walls and the tenon cheeks, not the end grain. Clamp across the width of the joint and check square before it sets.

Tips

Gauge everything from the marked face side. This single habit prevents most fit problems.
Cut the mortise first — adjusting a tenon is easy, adjusting a hole is not.
Aim for a joint that goes together with firm hand pressure. If it needs a mallet it will split the mortise walls when the glue swells the wood.

Common mistakes

Gauging from opposite faces on the two components — Any variation in thickness then appears as a step at the joint. Marking both parts from the marked face side makes the error cancel instead of doubling.
Sawing exactly to the line — Saw kerf and any wander take you inside it, and a loose tenon cannot be made bigger. Cut in the waste and pare down to a fit.

If it doesn't work

Tenon too loose

Cause: Sawn to the wrong side of the line — Fix: Glue a shaving of veneer to one cheek, let it dry, then pare back to fit.

Joint will not pull flush

Cause: Shoulders not square, or a bump on one — Fix: Pare the shoulder flat with a wide chisel; a slight undercut behind the visible edge helps it close.

Mortise walls blown out

Cause: Forced fit, or the mortise too close to the end — Fix: Leave extra length beyond the mortise and trim it after assembly. Pare the tenon rather than driving it.

Joint out of square when assembled

Cause: Mortise chopped at an angle — Fix: Pare the mortise walls to correct, and clamp a square block beside the next one as a sighting reference.

Questions people ask

How thick should the tenon be?

Traditionally about a third of the rail thickness, but in practice make it match the width of a chisel you own — the mortise is then a single chisel width and needs no paring sideways.

Which do I cut first?

The mortise. A tenon can be pared down to fit a mortise easily; a mortise cannot easily be made narrower to fit an undersized tenon.