spline

A spline is a strip of material, typically wood, that sits in matching grooves cut into two adjoining pieces to strengthen a glued joint. The spline bridges both parts, creating extra gluing surface and mechanical interlock that resists racking and separation. The grain of the spline usually runs perpendicular to the joint line, which provides cross-grain strength where it matters most.

Splines appear most often in frame-and-panel construction, edge-glued panels, and miter joints. In a frame corner, the spline runs across the miter; in a panel edge joint, it runs along the seam. The grooves are typically cut with a router or table saw to a depth of one-third to one-half the thickness of the wood, and the spline slides in loosely enough to allow clamping without binding. A standard spline width is 3/8 to 1/2 inch, though the dimension depends on the thickness of the stock being joined.

Grain direction and wood movement

The orientation of spline grain is critical. When grain runs perpendicular to the joint, the spline resists the racking forces that try to skew a frame out of square. However, because wood expands and contracts across the grain with changes in humidity, a spline that sits too tightly in its groove can cause checking or panel cupping. For this reason, a spline is usually made slightly thinner than the groove depth, or relief is left at one end to allow movement.

Miters and edge joints require different spline geometries. A miter spline often has a diamond or V-shaped cross-section to sit securely in the V-groove left by two 45-degree cuts. Edge splines are typically rectangular and uniform. Some woodworkers use contrasting wood or veneered splines as a visible detail, turning a structural joint into a design feature.

A poorly fitted spline causes joint failure: if too loose, it provides no support; if too tight, it prevents the glue from drawing the joint closed or causes the wood to split as it swells. The spline must be dry when inserted, otherwise moisture migration after gluing can swell it and open the joint at the glue line. High-density woods like maple or holly work best for splines because they are stable and resist compression.