Types of Wood Joints: What Each One Does and Why

Wood glue holds long grain to long grain almost perfectly and holds end grain to almost nothing. The USDA Forest Products Laboratory puts a number on it: even with special techniques, butt joints reach only about 25% of the tensile strength of the wood parallel to the grain, because adhesive overpenetrates into the open lumens of end grain and leaves voids in the glue line.

So joinery is not decoration. The handbook states the design rule directly - it is necessary to design these joints with interlocking surfaces so that edge grain of one piece bonds to edge grain of the adjoining piece, and increasing the joint surface area helps by providing more bondline to transfer load. Every joint below is a different way of obeying that one rule.

This is the corner-and-rail family: the end of one board meets the face or edge of another. The Wood Handbook illustrates the group as plain, miter, dowel, mortise and tenon, dado tongue and rabbet, slip or lock corner, dovetail, blocked, and tongue-and-groove.

• [Mortise and tenon](/lookup/wood-guides/mortise-and-tenon-joint) - a tongue in a socket. The reference joint for chairs, tables and doors.
• Dowel - round loose tenons. Faster to make, measurably weaker.
• [Dovetail](/lookup/wood-guides/dovetail-joint) - angled tails and pins that resist being pulled apart in one direction mechanically, with or without glue.
• [Dado](/lookup/wood-guides/dado-joint) and rabbet - a trench or a stepped edge that locates a shelf or a case side and adds long-grain contact.
• Miter - a 45 degree cut that hides end grain and has almost no strength on its own, so it is normally splined, keyed or reinforced.

The handbook adds a warning that applies to all of them: because wood swells much more across the grain than along it, moisture changes produce large internal stresses in end-to-edge-grain joints, and they should be protected from changes in moisture content in service.

The second family joins end to end. The handbook illustrates it as butt, plain scarf, vertical structural finger joint, horizontal structural finger joint, and nonstructural finger joint - and it is blunt that a plain butt joint is practically impossible to make strong enough for ordinary service with conventional bonding.

The way out is geometry. To approximate the tensile strength of clear solid wood, a scarf or finger joint should have a surface area at least ten times the cross-sectional area of the piece, because wood is roughly ten times stronger in tension than in shear. Joints cut at a slope of 1 in 12 or flatter give the highest strength; in plywood scarf and finger joints a slope of 1 in 8 is typical for structural products. A well-manufactured scarf, finger or lap joint in end grain can reach up to 90% of the tensile strength of clear wood, though its cycles-to-failure under repeated loading are often lower.

Matthias Wandel of Woodgears built a test stand to break chair-scale joints and published the numbers. Rails were 6 x 1.7 cm into a 3.5 x 3.5 cm post, tenons 10 mm thick, 27 mm deep and 55 mm wide, assembled with yellow carpenter's glue, load applied 20 cm from the post.

In spruce, mortise and tenon joints failed at 170, 175, 140, 180, 185 and 190 lb, averaging 172 lb; dowel joints failed at 120, 140 and 145 lb, averaging 135 lb; pocket-hole joints failed at 115, 110 and 140 lb. In maple, mortise and tenon reached 300 and 270 lb, dowels 230 and 245 lb, a pocket-hole joint 220 lb.

Two lessons hold up. The mortise and tenon beat dowels by only about 25%, not by the margin folklore suggests. And tenon fit, from hammer-tight to slightly loose, made no detectable difference as long as glue reached everywhere - which matches the handbook's point that the bondline, not the friction, carries the load.

• Chair and table frames, doors: mortise and tenon. Highest measured strength in the rail-to-post test and the joint with the longest service record.
• Drawer fronts and fine box corners: dovetail, for mechanical resistance to being pulled apart plus a large glue area.
• Plain box and drawer corners cut on a machine: box joint, which trades the dovetail's angle for many more glue surfaces.
• Shelves in a carcase, cabinet backs: dado and rabbet, which locate the part and stop racking.
• Shop furniture, jigs, face frames, anything you may take apart: pocket-hole screws or a lap joint.
• Making a board longer: scarf or finger joint, never a plain butt.

If none of them fits, ask which grain surfaces the joint is asking the glue to hold, then redesign until the answer is long grain to long grain.

Lists of "12 types" or "15 types" of wood joints draw from the same pool. Here it is with one line each, drawn from Wikipedia's joinery article and the UK joinery firm JCM Contracts.

• Butt - one piece butted against another; the simplest and weakest joint.
• Miter - both pieces bevelled, usually at 45 degrees, to meet at 90 degrees; used on frames and visible corners.
• Dowel - a butt joint reinforced with dowel pins.
• Biscuit - an oval wooden biscuit glued into matching crescent-shaped slots cut with a small circular blade; mainly for aligning edge joints.
• Pocket hole - a screw driven at an angle, usually about 15 degrees, from one piece into the other. Pocket holes
• Rabbet - a step cut along an edge that another piece seats into; used for backs, glass and panels.
• Dado - a slot across the grain that another piece sits in. Dado
• Tongue and groove - a ridge on one edge fits a groove on the next; flooring and panelling.
• Lap and half lap - one piece laid over the other, with half the thickness removed from each in a half lap. Lap joint
• Bridle - an open mortise and tenon, forked on one side.
• Box or finger - interlocking square fingers at a corner. Box joint
• Dovetail - angled pins and tails that resist being pulled apart. Dovetail
• Mortise and tenon - a tenon fitted into a hole cut for it. Mortise and tenon
• Scarf - bevel cuts that join two members end to end.

They do three jobs. Most join the end of one piece to the edge or face of another; tongue and groove and biscuits mostly join boards edge to edge; and the scarf, like the structural finger joint in the section above, lengthens a board end to end.

Wikipedia's joinery article notes that some joints use mechanical fasteners, bindings or adhesives, while others use only wood. In practice every joint on this page relies on one or more of three things:

• Glue on long grain - the main method in furniture, and the reason interlocking shapes exist; the Wood Handbook's rule is to present edge grain to edge grain and increase the glued area.
• Loose inserts - dowels, biscuits and splines add glue surface and alignment to a simple butt or miter.
• Mechanical fasteners and wood locks - screws in pocket holes, bolts in bed frames, and wooden pegs or wedges through a tenon, which hold without relying on glue and allow the joint to be taken apart or to survive glue failure.

No standard measures difficulty, so judge it by how many surfaces must fit at the same time and whether they are cut by hand. By that test the hand-cut dovetail is the most demanding common joint - every pin and tail must match its partner, and a half-blind dovetail adds a socket that cannot be seen or adjusted from outside. Angled and compound mortise and tenons, as in chairs with splayed legs, are next, because the mortise and the tenon shoulders must agree in two angles at once. Butt, pocket-hole and biscuit joints are the easiest because a jig or machine sets the geometry.