How the joint works
The tenon's cheeks run along the grain of the rail, and the walls of the mortise run along the grain of the stile, so the glue lines inside are long grain to long grain. The Wood Handbook names the mortise and tenon among the joints that turn an end-to-edge connection — otherwise one of the weakest to glue — into an edge-to-edge bond by interlocking. The shoulders, which butt against the stile, set the rail's length and resist racking. The parts are drawn and named in BenchRoot's joint anatomy; how it compares with other joints is in choosing a wood joint.
| Tenon | Mortise |
|---|---|
| Length | Depth — a little deeper than the tenon is long |
| Width | Length |
| Thickness | Width |
From Leigh's FMT Pro user guide, which notes that the two parts' dimensions do not share names.
The variants
| Variant | What changes | Used for |
|---|---|---|
| Blind (stub) | The tenon stops inside the stile and does not show | Most furniture frames |
| Through | The tenon passes through and shows on the far face | Where the joint is a feature, or is to be wedged |
| Haunched | A short step on the tenon fills a groove run through the stile | Frame-and-panel doors with through grooves |
| Wedged through | Wedges driven into the tenon's end spread it in a flared mortise | Decorative and mechanical locking |
| Twin or double | Two tenons side by side or in line instead of one wide one | Thick or wide parts |
| Loose | A separate tenon glued into mortises in both parts | Machine-cut joinery; parts with no spare length for a tenon |
| Pinned | A pin through cheek and mortise wall | Japanese joinery uses pins (sen) and keys (shachi) |
Haunch, wedging and twin tenons from Leigh's guide; the Japanese forms, including double and quadruple tenons and the wedged 'fox' tenon, from JAANUS.
Leigh notes that haunched tenons are less common than they were, because a router can stop the panel groove short of the joint and remove the need for one. A tenon set at an angle is weaker, too: Leigh says its strength across the grain falls considerably as the angle increases. For Japanese tenon forms, and the pins and keys that lock them, see Japanese joinery.
Proportion and fit
- Fit: Leigh's best fit for its routed joints is about 0.005 in loose: the dry joint should push together fairly easily but not fall apart under its own weight. If a mallet is needed, it is too tight. A hand-cut joint is judged the same way — see wood joint techniques.
- Depth: cut the mortise slightly deeper than the tenon is long — Leigh gives 1/32–1/16 in — so the shoulders close against the stile before the tenon bottoms out.
- Thickness: by hand, the width of the mortise chisel sets the mortise and so the tenon; BenchRoot's joint anatomy starts from a tenon about a third of the stock's thickness. See mortise chisels and the mortise gauge in marking gauges.
- Width and movement: a tenon's width runs across its rail's grain, so a wide tenon shrinks and swells across the mortise's length. The Wood Handbook warns that moisture changes in end-to-edge joints set up large stresses; a haunch or twin tenons keep each tenon narrower.
- Cutting: cheeks and shoulders with a tenon or carcass saw — see backsaws — and the mortise with a chisel or a drill and chisel.
Planning a mortise and tenon
- Blind or through; haunched if a groove runs through
- Tenon thickness set by the chisel or bit you will use
- Mortise depth a little more than the tenon's length
- Width kept moderate, or split into twin tenons
- One reference face for both parts
- A dry fit that pushes together by hand
Sources
Fit and depth figures are Leigh's for joints routed on its jig; a hand-cut joint is judged by the same feel. No source reviewed sets a universal ratio of tenon to stock thickness, so none is stated as a rule.
- 1.
FMT Pro Frame Mortise & Tenon Jig User Guide. Leigh Tools (a Lee Valley Tools company).
Used for: The terms that pair tenon and mortise — tenon length and mortise depth, tenon width and mortise length, tenon thickness and mortise width; the best fit for its routed joints about 0.005 in loose, pushing together fairly easily without falling apart, a joint needing a mallet being too tight; mortises routed 1/32–1/16 in deeper than the tenon so the shoulders seat; haunched tenons filling a groove run through the stile, now less common because stopped grooves avoid the need; wedged through tenons as a decorative touch; a tenon's strength across its grain falling considerably as its angle increases; dowels where the strength of a mortise and tenon is not required.
- 2.
Chapter 10: Wood Adhesives: Bond Formation and Performance. USDA Forest Service, Forest Products Laboratory — Wood Handbook, FPL-GTR-282, 2021.
Used for: The mortise and tenon among the end-to-edge joints that interlock so edge grain bonds to edge grain; moisture changes in such joints producing large internal stresses, so they should be protected from changes in moisture content.
- 3.
hozo ほぞ (glossary entry). Japanese Architecture and Art Net Users System (JAANUS).
Used for: Hozo, the tenon, and its Japanese forms: single, double and quadruple tenons, haunched, stacked, dovetail and wedged ('fox') tenons.
- 4.
shiguchi 仕口 (glossary entry). Japanese Architecture and Art Net Users System (JAANUS).
Used for: Tenoned angle joints often reinforced by pins (sen) or long keys (shachi).
About this reference
Written by BenchRoot Editorial. How BenchRoot sources, checks and corrects its pages is set out in the Editorial Policy. If something here is wrong or unclear, the contact page explains how to tell us.