Every project has a few places where one board meets another, and the joint you choose decides how strong the piece is, how it looks and how long it takes. The joinery database lists twenty, rated for strength and difficulty. In practice a handful do most of the work.
What a joint must do
Glue holds best when it grips long grain to long grain. End grain absorbs glue and gives a weak bond. So joints are built to add long-grain surface, or they are reinforced with a mechanical element such as a fastener, dowel or interlocking shape. Strength is also about resistance to forces in the direction the piece will be loaded: a joint that is solid against a pull may fail if twisted.
The simplest joints
Butt joint. The end or edge of one board meets the face of another, held with glue and screws or nails. It is quick and needs few tools. Strength is low to medium: the glue is on end grain, so the fasteners do most of the work. It is fine for boxes, framing and plain shelves.
Mitre joint. Two ends cut at 45 degrees make a 90 degree corner with no end grain showing. It looks neat on picture frames and trim but is also a weak glue joint, since both faces are end grain. Reinforce with splines, biscuits or small keys across the corner.
Edge joint. Two boards glued edge to edge, long grain against long grain, to make a wide panel. It is strong: a well-fitted edge joint is as strong as the wood itself. It is the basis of tabletops and panels.
Joints with added strength
Pocket screw joint. A screw driven at an angle through a pocket hole joins the end of one board to the face of another. It is quick, needs only a jig and a drill, and is fairly strong. The holes show unless hidden. Common for face frames and cabinets.
Dowel joint. Round pins fit into matching holes in both boards and increase the glue area. They need accurate drilling, with a jig or marking points, to line up. Medium strength.
Biscuit joint. An oval compressed-wood biscuit swells in a slot cut in both parts with a biscuit joiner. It helps alignment and adds some strength. Good for panels and carcase corners.
Loose tenon. A separate tenon fits into mortises cut in both boards. It gives the strength of a mortise and tenon with less work, because the same machine cuts both mortises.
Joints that interlock
Half-lap. Half the thickness is removed from each board so they overlap flush. Strong, because the glue area is large and the parts interlock. Used in frames and crossing rails.
Dado, groove and rabbet. A dado is a channel across the grain that holds the end of another board, such as a shelf in a side. A groove runs with the grain. A rabbet is a step cut along an edge. They locate a part so it cannot move sideways. A dado is the usual way to hold a bookcase shelf, giving strong support that a fastener alone could not match. The rabbet is used to set the back panel into a cabinet.
Tongue and groove. A tongue on one edge fits a groove in another. It aligns boards for floors, panelling and doors.
Mortise and tenon. A rectangular tenon on the end of one board fits into a mortise in the other. It is among the strongest joints for frames, table legs and chairs, because it has large long-grain surfaces. It takes practice to cut accurately.
Dovetails. Interlocking tails and pins make joints that resist being pulled apart. Through dovetails show on both faces, half-blind dovetails hide on the front of a drawer, and sliding dovetails join a shelf to a side. They are a mark of fine work, and they take the longest to cut. Box joints, or finger joints, have square fingers instead of angled ones and are easier, with good glue area.
Choosing by project
| Project | Joint |
|---|---|
| Plain box | Butt with glue and screws, or rabbet |
| Picture frame | Mitre with splines |
| Tabletop | Edge joints |
| Table legs to rails | Mortise and tenon, or pocket screws |
| Bookshelf | Dado for shelves |
| Drawer | Half-blind dovetail or box joint |
| Face frame | Pocket screws or dowels |
| Door frame | Mortise and tenon, or half-lap |
Strength versus effort
A pocket screw joint takes a few minutes and is strong enough for most household pieces. A mortise and tenon takes an hour to cut and will outlast the wood around it. Choose the quickest joint that is strong enough for the load, and use the time saved on the parts where strength matters most. The fastener database gives screw sizes with pilot hole diameters for hardwood and softwood, which is useful for pocket and butt joints.
Identify a joint you find
The joint identification tool takes a photo of a joint and returns its name, how it is made and how strong it is, which helps when you are copying a piece of furniture or fixing one.