Spline joint and splined miter
A spline is a thin strip of wood glued into matching slots in two pieces, across the joint between them. In an edge joint it lines boards up; in a mitred corner it bridges the joint. Rockler notes that a plain mitre connects end grain to end grain, so it is not very strong without reinforcement, and that splines add strength.
Why a mitre needs help
The USDA Wood Handbook says a plain end-grain butt joint cannot be glued strong enough for ordinary service, and that where end grain meets edge grain the joint needs interlocking parts, so that edge grain is glued to edge grain. It names dowels, mortise and tenons and rabbets as examples of strong joints of that kind.
One source: USDA Forest Products Laboratory, the authority for wood adhesive joints it tested
Two kinds of spline
In an edge or mitre spline, the slots run along the length of the joint, inside it, and the spline is hidden. In a key or feather spline, saw kerfs are cut across the outside corner of a glued-up mitred box or frame and thin keys are glued in, often in a contrasting wood. Rockler sells a router table jig that holds a box at 45° to cut those slots.
Grain and fit
A spline is cut so its grain runs across the joint line, not along it, so it can’t split along the joint. It should slide into its slots with hand pressure. For key splines, the slot is cut with a table saw blade or a router bit and the keys are planed or sanded to the kerf’s width, then trimmed flush once the glue sets.
Related joints
Sources
- Rockler, Woodworking furniture joinery: butt joints, mortise and tenon, dovetails, read 2026-11-07
- USDA Forest Products Laboratory, Wood Handbook (FPL-GTR-282, 2021), ch. 10 "Wood Adhesives", End-Grain and End-to-Edge-Grain Joints, read 2026-11-07
Proportions marked as a traditional rule are shop tradition, not measurements. This site gives no joint-strength numbers: the published tests each used a different rig. How we source these numbers.