What a joint does
A joint connects two pieces of wood so the assembly holds together under the loads it will face. A good joint offers enough glue surface or mechanical locking, resists the direction it will be stressed in, and is within your skill and tools to make accurately. The simplest joint that does the job is often the best choice.
Safety: joinery involves sharp saws and chisels. Clamp work firmly, keep hands behind the cutting edge, and wear eye protection. Practice on scrap before cutting good boards.
Grain decides strength
Before looking at individual joints, remember why glued joints work. Wood glue forms strong bonds on long-grain surfaces and weak ones on end grain, which soaks up glue like a bundle of straws. So a joint that glues long grain to long grain, such as an edge joint, is typically strong, while one that glues end grain to anything is weak unless it has extra help. This is a recurring theme, and the article on reading wood grain explains why. The mechanics of gluing are in the article on wood glue.
Wood also swells and shrinks across its width as humidity changes. Joints that lock wide boards rigidly across their grain can crack, so some designs need to leave room, as the piece on wood movement explains.
Butt joint
The simplest joint: one piece meets another with two flat faces and nothing else. It is easy to make but, held by glue alone, the glued area is often end grain and therefore weak.
Best for: quick frames, rough boxes, and anything where fasteners or other reinforcements will carry the load.
Make it stronger with screws, nails, dowels, corner blocks or metal brackets. For a choice of fasteners, see screw types.
Edge joint
Two boards are glued along their long edges to make a wider panel. When the edges are planed straight and square and clamped properly, the joint can be very strong, because it joins long grain. It is the standard way to make tabletops and panels from narrower boards.
Best for: wide panels such as tabletops, shelves and cabinet sides.
Lap joint
In a lap joint, material is removed from each piece so they overlap and sit flush. A half-lap removes about half the thickness from each. The overlap gives a good glue surface and resists twisting.
Best for: frames, such as a door or a garden trellis, and bracing where two pieces cross.
Tip: make sure the shoulder cuts match. Marking from the actual mating piece, as covered in marking out, is more reliable than measuring.
Rabbet joint
A rabbet is a step cut along an edge or end. It supports another piece that fits into the step. It adds glue surface and helps locate parts during assembly.
Best for: box corners, drawer sides and cabinet backs set into the sides.
Dado (housing) joint
A dado is a groove cut across the grain into which another piece fits. A shelf set into a dado is supported along its whole width, which resists sagging and keeps the shelf from sliding. A groove running along the grain is usually called a groove instead of a dado.
Best for: shelves, bookcase dividers and drawer bottoms.
Dowel joint
Round pegs fit into matching holes drilled in both pieces. Dowels help align parts and add a little resistance to side loads, but the strength depends on accurate drilling. Misaligned holes lead to gaps and stress.
Best for: joining boards edge to edge for alignment, attaching rails to legs in lightweight furniture, and reinforcing butt joints.
Tip: use a drilling guide or doweling jig and mark carefully. Even small errors in hole position can stop pieces from closing.
Mortise and tenon
A tenon is a projecting tongue on one piece that fits into a mortise, a pocket cut in another. It is a traditional furniture joint with a large long-grain glue surface and mechanical resistance to racking forces, which makes it strong for frames, chair parts and table legs.
It takes more skill than the joints above, so a beginner should expect to practice. Cutting the tenon with a saw and the mortise by chopping with a chisel is achievable with basic hand tools if the chisels are sharp; see sharpening chisels and plane blades.
Best for: table and chair frames, doors, and any assembly that must resist twisting.
Miter joint
Two pieces are cut at matching angles, commonly 45 degrees for a square corner, so that no end grain shows on the outside. They look neat but glue poorly, because the angled surfaces expose end grain. A miter often benefits from reinforcement such as splines (thin strips set in a slot across the joint), keys or biscuits.
Best for: picture frames and decorative corners.
Choosing a joint
A few questions help:
- What forces will the joint face? Tension, compression, racking (sideways twisting) and sag each favor different joints.
- How much long-grain glue surface does it have? More generally means stronger.
- Is it visible? Some joints are decorative as well as structural.
- Do you have the tools and skill? An accurately cut simple joint beats a sloppy complex one.
- Can it move with the wood? Wide panels may need to float.
Fit, glue and clamp
Almost all joints rely on a good fit. A gap invites a weak glue line, and forcing a too-tight joint together can split the wood. Dry fit before gluing, apply glue sparingly to the mating surfaces, and clamp with even pressure while it sets, following the glue manufacturer’s directions. Check the assembly for square before the glue sets.
A learning path
A sensible order might be butt joints with screws, then edge joints, rabbets and dados, then lap joints, dowels and finally mortise and tenon. Each builds the skills needed for the next, and each shows up repeatedly in basic woodworking projects and in building furniture. Make a few extra practice joints from scrap, break one on purpose to see how it fails, and you will learn more than from any diagram.