Start with the grain that meets
The Wood Handbook sorts glued joints by the grain on each side of the glue line, and that is the most useful way to choose one. Long grain glued to long grain — boards edge-joined into a panel — bonds well when the surfaces meet flat. End grain is the problem: it is so porous that glue soaks away from the line, and the Laboratory says end-grain butt joints are practically impossible to make strong enough for ordinary service, reaching only about 25% of the wood's tensile strength even with special techniques. A joint earns its strength by turning end grain into edge grain. A scarf or end-to-end finger joint does it with a long slope; a mortise and tenon, a dowel, a rabbet or a dovetail does it by interlocking, so that the cheeks and faces inside the joint bond long grain to long grain.
- More glue area is not always more strength. The Handbook notes that shaped edge joints such as tongue and groove do not usually test stronger than a flat joint, because the shaped faces cannot be machined to meet perfectly; their value is alignment during clamping.
- A joint that interlocks also holds mechanically. Tails resist being pulled out one way; a pinned or wedged tenon resists withdrawal without relying on glue alone. Which direction the load comes from decides which interlock is worth cutting.
Allow for movement across the grain
Wood barely moves along its length — the Handbook gives about 0.1–0.2% from green to ovendry — but moves many times that across it, most of all tangentially, with radial movement about half the tangential. A joint that fixes a wide board across the grain of another fights that movement every season. The Laboratory says moisture changes in cross-grain joints produce large internal stresses and that such joints should be protected from changes in moisture content. In practice that is why panels float in grooves, tabletops are fastened so they can move, and wide cross-grain glue joints are avoided. How much a board will move is a calculation, not a guess: see wood movement.
The joints and what each is for
| Joint | Grain that meets | Resists | Typical use | Guide |
|---|---|---|---|---|
| Butt | End to edge | Little on its own | Hidden parts, with fasteners or reinforcement | — |
| Rabbet and dado | End or edge into a housing | Racking, with a shoulder to locate it | Carcase backs, shelves | Dado blades |
| Mortise and tenon | End to edge, interlocked | Racking and withdrawal, more when pinned or wedged | Frames, chairs, tables, doors | Mortise and tenon |
| Dovetail | End to end at a corner, interlocked | Pull in one direction | Drawers, boxes, carcases | Dovetails |
| Box (finger) joint | End to end at a corner, interlocked | Racking; relies on glue on many cheeks | Boxes, drawers, cases | Box joints |
| Scarf | End to end, on a slope | Tension along the length | Lengthening a member | — |
| Japanese tsugite and shiguchi | End to end, or at an angle | Mechanical locking, often pinned or keyed | Timber framing, furniture | Japanese joinery |
Grain families from the Wood Handbook, chapter 10; the Japanese division from JAANUS. The joints are drawn in section in BenchRoot's joint atlas.
Appearance is a legitimate reason to choose a joint. Veritas, which makes guides for cutting dovetails by hand, says plainly that modern glues and joinery techniques can make joints as strong as dovetails or stronger, and that dovetails are still chosen for how they look and for the challenge. Where the strength of a mortise and tenon is not needed, Leigh suggests dowels can serve. Japanese carpentry has its own vocabulary for the same choices — see Japanese joinery and the lattice craft of kumiko.
How you will cut it
- By hand: a saw, chisels, a marking gauge and a square cut every joint here. See backsaws, Japanese saws, chisels, mortise chisels and marking gauges.
- On the table saw: rabbets, dados and box joints with a dado set or box joint cutter, where your saw's manual allows them — see dado blades and table saw safety.
- Whatever the tool, the same techniques make a joint fit: reference faces, knife and gauge lines, cutting on the waste side and a dry fit. They are in wood joint techniques; the first dovetail is in cutting dovetails by hand.
- To practise on a real piece, build a dovetailed box or a kumiko lattice panel.
Choosing a joint
- Which grain meets: edge, end or both
- Which way the load comes: pull, racking, weight
- Whether the joint crosses the grain of a wide part
- Whether it will show, and how it should look
- The tools you have to cut it
- Clamps to close it — see clamps
Sources
Joint behaviour is from the Wood Handbook (2021). Strength figures there are for glued joints tested as the Laboratory describes, not ratings of joints in furniture; no source reviewed ranks joints by strength in general, so none is ranked here.
- 1.
Chapter 10: Wood Adhesives: Bond Formation and Performance. USDA Forest Service, Forest Products Laboratory — Wood Handbook, FPL-GTR-282, 2021.
Used for: Joints grouped by the grain they bond — edge to edge, end to end, end to edge; end-grain butt joints practically impossible to make strong, reaching only about 25% of the wood's tensile strength even with special techniques; scarf and finger joints converting end grain to edge grain with a surface at least ten times the cross-section; end-to-edge joints made to interlock so edge grain bonds to edge grain (dowels, mortise and tenons, rabbets, dovetails among the examples); shaped joints not stronger in practice than flat ones; moisture changes in cross-grain joints producing large internal stresses.
- 2.
Chapter 4: Moisture Relations and Physical Properties of Wood. USDA Forest Service, Forest Products Laboratory — Wood Handbook, FPL-GTR-282, 2021.
Used for: Wood moving least along the grain (about 0.1–0.2% from green to ovendry) and most tangentially, with radial movement about half the tangential.
- 3.
Dovetail Saw Guides — instructions (INS-622_B). Veritas Tools Inc. (Lee Valley), 2018.
Used for: Veritas's statement that modern glues and joinery techniques can make joints of equal or greater strength than dovetails, which remain desirable for their appearance.
- 4.
FMT Pro Frame Mortise & Tenon Jig User Guide. Leigh Tools (a Lee Valley Tools company).
Used for: Leigh's note that doweling may suit where the strength of a mortise and tenon is not required.
- 5.
tsugite 継手 (glossary entry). Japanese Architecture and Art Net Users System (JAANUS).
Used for: The Japanese division of joints into tsugite, which join timbers end to end, and shiguchi, which join them at an angle.
- 6.
shiguchi 仕口 (glossary entry). Japanese Architecture and Art Net Users System (JAANUS).
Used for: Shiguchi as angle joints, including tenons, laps, mitres and dovetails, often reinforced with pins or keys.
About this guide
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