The mortise and tenon joint
The mortise-tenon is the load-bearing joint of furniture construction. A rectangular peg (the tenon) fits into a matching socket (the mortise). When well-fitted and glued, it resists racking, tension, and shear. Chair legs, table aprons, door frames, and cabinet face frames all rely on it.
Proportions
The tenon thickness should be one-third of the stock thickness — a 25 mm thick rail gets a roughly 8 mm tenon. This balances the cheek walls of the mortise (which provide strength) against the tenon itself. The tenon length is typically 2–2.5 times its thickness, so 16–20 mm for an 8 mm tenon.
Haunched tenons — where a short shoulder fills a groove — are used where the joint meets the end of a stile, such as in a frame-and-panel door. The haunch prevents the stile end from twisting outward.
Marking out the mortise
Set a mortise gauge to the width of the chisel you will use. Transferring chisel width directly eliminates conversion errors. Mark both faces from the same reference edge, which keeps the mortise centred even if your gauge is set slightly off-centre.
Score the shoulder lines with a marking knife, not a pencil. A knife line creates a small shoulder that the chisel can register against, giving a cleaner wall.
Chopping the mortise
Work from both ends toward the middle to avoid blowing out the far end. Start about 2 mm in from the end line; work back to it last. Strike the chisel firmly with a mallet — the goal is to sever fibres cleanly, not scrape them away. Clear waste frequently.
For long mortises in oak, a brace and bit to remove the bulk of the waste before chiselling saves considerable time and reduces fatigue. Drill a series of holes slightly smaller than mortise width, then pare the walls clean.
Cutting the tenon
Saw the cheeks first, then the shoulders. When sawing cheeks, start the cut with the board tilted at 45°, establishing the kerf on two faces before returning the board to vertical. This helps the saw track on both sides simultaneously.
Leave the tenon slightly fat — a few tenths of a millimetre — and pare or plane it to final fit. A properly fitted tenon slides in with hand pressure and produces a faint squeak. Too loose means glue alone carries the load; too tight risks splitting the mortise walls.
The dovetail joint
Dovetails are used primarily for drawer construction and box-making. The mechanical interlock of the angled pins and tails means the joint resists pulling apart in one direction without relying on glue. In dry workshop conditions, a well-fitted dovetail does not technically need glue at all.
Tails first or pins first?
Both sequences produce identical joints. Tails-first is the more common teaching sequence: cut the tails, use them as a template to scribe the pins, cut the pins. The advantage is that scribing directly off the tails eliminates accumulated measurement error — you are fitting to the actual piece, not to a number.
Pins-first is less common in hand-tool practice but preferred by some because the pins are smaller and more difficult to cut; doing them when the saw hand is fresh can produce cleaner results.
Dovetail ratio
The ratio of the dovetail angle is typically 1:6 for softwood (a steeper angle, more mechanical resistance) and 1:8 for hardwood. In Polish oak and beech, 1:8 is standard. Steeper angles in hardwood create short grain at the tail corners that snaps under stress.
Mark the angle with a sliding bevel, not freehand. Consistency matters more than the exact ratio — varying the angle across a set of tails reads as sloppiness even if each cut is individually clean.
Sawing technique
A dovetail saw — a backsaw with fine teeth (14–16 tpi) — is the right tool. Hold the saw lightly; grip causes the wrist to tense and the saw to wander. The cut should feel almost passive. Start the kerf with a backward stroke, then take full strokes.
Saw to the waste side of the line, leaving the line itself on the piece. That half-line of wood is removed at the bench with a sharp chisel in the fitting stage.
Fitting the joint
After sawing, remove the bulk of the waste between tails with a coping saw or fret saw, staying a millimetre above the baseline. Then chop the baseline with a chisel — always working halfway through from each face to avoid blowing out the back.
When fitting pins to tails, pare a small chamfer on the inside faces of the pins to act as a lead-in. Assemble dry, mark any tight spots with chalk or a thin coat of pencil graphite, disassemble, pare the marked areas, and try again.
Tool requirements for both joints
For mortise-tenon
Mortise gauge, marking knife, try square, mortise chisels (6 mm, 8 mm, 10 mm), mallet, tenon saw, shoulder plane for fitting cheeks.
For dovetails
Marking gauge, sliding bevel, marking knife, dovetail saw, coping or fret saw, bench chisels (3 mm, 6 mm, 12 mm), wooden mallet.
Glue-up and clamping
PVA (white or yellow woodworking glue) is the standard for both joints in a dry interior environment. Apply glue to all mating faces — the mortise walls, tenon cheeks and shoulders, and all pin and tail surfaces. Avoid pooling glue at the bottom of the mortise, which creates hydraulic pressure and can split the component.
For mortise-tenon joints, clamp across the shoulders, perpendicular to the joint axis. For dovetails, light pressure perpendicular to the tails is sufficient; heavy clamping distorts thin drawer sides.
Check for square by measuring the diagonals before the glue sets. A difference of more than 1 mm in a 400 mm frame requires correction before the glue skins over.
