Traditional Joinery: Mortise and Tenon, Dovetails and Housings

A table looks immaculate: square corners, fine finish. A year later it wobbles, black lines open in the frame corners, and a leg works loose. The cause is not the glue or hand skill, but that the chosen joint never had the capacity for the loads placed on it. Joint selection is a structural decision, not a styling detail.

1. Where Joint Strength Comes From

• Glue surface area: modern wood adhesive is stronger than the wood fibre itself, on one condition: long-grain to long-grain contact over sufficient area.
• Mechanical interlock: the joint's shape resists movement. A tenon in a mortise resists racking; a dovetail locks absolutely in one direction even without glue.

Why End Grain Will Not Glue

End grain is like a bundle of cut tubes. Glue wicks into them, the surface layer starves, and the joint is left with little effective adhesive. So any joint relying mainly on end-grain gluing is inherently weak, and that is where the classification begins.

2. Butt Joints and Their Inevitable Weakness

The end of one piece meets the face or end of another. Glue area is poor because one surface is end grain, and there is zero mechanical interlock. Any bending moment works on a very short lever arm and pulls it apart. Acceptable only in hidden, non-structural work where mechanical fasteners actually carry the load, or in temporary work. In a door frame or chair it fails whatever the glue.

3. Mortise and Tenon

The parent joint of frame construction: chairs, tables, doors, windows. A rectangular mortise in one piece receives a projecting tenon from the other, combining large long-grain glue area on four tenon faces with interlock that resists racking.

Proportions

• Tenon thickness: roughly one third of the stock thickness. Thinner and the tenon breaks; thicker and the mortise walls weaken and the wood splits around them.
• Tenon length: longer gives more glue area and better racking resistance. Mortise depth is usually slightly greater than tenon length to take excess glue.
• Tenon width: very wide tenons are better split into twins, reducing wood movement and improving clamping.

The Shoulder's Function

The shoulder is the flat face around the tenon that bears on the other piece. It is routinely underrated: it sets the angle, stops rotation, and transfers compression loads. An uneven or gappy shoulder means a joint that wobbles even with a tight tenon. The shoulder matters more than tenon tightness.

4. Dovetails

Interlocking angled pins and tails, used at drawer and box corners. The angled geometry forms a mechanical lock in one direction that cannot be pulled apart — exactly the direction a drawer is pulled every day. Through dovetails suit utility work and corners where the craft is meant to show; half-blind dovetails hide the joint at a drawer front. Execution needs accurate angles and spacing, and excessive angle produces fragile short-grain tips.

5. Housing / Dado Joints

A groove the width of the shelf, part way into the thickness of the upright, receives the shelf end. The basic shelving joint: it supports the shelf mechanically from below so the load does not rest on glue, sets position and angle automatically, and stops the shelf rolling while helping square the whole carcass. Depth is typically around a third of the upright's thickness; deeper weakens the side, shallower gives too little interlock. It can run through the edge or be stopped short to hide it at the front.

6. Half-Laps and Mitres

Half-lap: half the thickness is removed from each piece at the crossing so they interleave to a single thickness. Large long-grain glue area, simple to cut, excellent for back frames and mid-span crossings — but it halves each member's section, so keep it out of heavily loaded parts.

Mitre: two angled cuts make a clean corner with no visible end grain, visually the best option for frames and mouldings. Structurally it has two problems: the glue surface is close to end grain, so adhesion is limited; and grain runs at right angles across the joint, so the two pieces move in different directions with humidity and the corner tip opens — the familiar gap in old picture frames. The fix is not more glue but added interlock: a hidden tenon, a spline across the corner, a biscuit, or a rear mechanical fastener.

7. Biscuits and Dowels as Alignment Aids

• Biscuits: primarily for alignment and anti-slip during clamping. They add moderate strength and come nowhere near a mortise and tenon in racking resistance.
• Dowels: more genuine interlock than biscuits, entirely dependent on hole accuracy. A slightly misplaced hole stops the joint closing and leaves permanent stress.

Both are perfectly sound in manufactured boards and cabinetwork, but they do not replace a structural joint in a frame taking dynamic loads such as a chair.

8. Selection Table

9. Grain Direction and Clamping

Track grain direction in both pieces. If grain runs at right angles across a long glue line, wood movement fights the glue and may split one part — exactly the problem with gluing a wide solid shelf across the grain onto an upright. Design joints that permit calculated movement instead.

• Clamping aims at full contact and a thin, even glue layer, not at squeezing all the glue out.
• A light, even squeeze-out around the joint is a good sign of adequate glue and pressure. No squeeze-out usually means too little glue or premature drying.
• Excessive pressure on a tight joint can starve it and leave a weak glue line, especially with thin-bodied adhesive.
• Spread clamps along the joint, and check squareness by measuring diagonals while tightening — a joint that dries out of square cannot be corrected later.
• Remove excess glue at the right moment for its type; dried glue blocks finish and shows as pale patches under varnish.

Conclusion

Choose the joint by its load, not by ease of cutting. Mortise and tenon for frames under racking, dovetails for drawers, housings for shelves, hidden interlock for mitres. The principle behind all of it: glue needs long grain and area, and the joint needs a shape that resists movement. When both are present, the joint lasts a lifetime.