A spline is a strip of wood, plywood, or other material (such as Masonite), inserted into matching grooves or plows, along the edges of two boards. The purpose is to reinforce and align the edges. A spline can be used as a substitute for the tongue and groove.

The Basics:
Splines for strength: Splines are often used to reinforce miter and butt joints
Splines as accents: Dress up a joint by using contrasting woods
Anatomy of the joint: Use the correct proportions to ensure strength and stability

Splines for strength
When used on a miter joint, the spline adds extra strength to an otherwise delicate joint. The spline is also useful because it helps to align the two joining parts during assembly and glue-up. For extra strength, the grain direction of the spline can run at 90° to the grain of the boards.

Splines as accents
Another advantage is that a contrasting wood spline can be used as an accent. Or a spline can be “capped” with small contrasting end pieces.

Anatomy of the joint
The dimensions of a spline are similar to the dimensions of a tongue-and-groove joint. If the working stock is 3/4 in. thick, the spline should be 1/4 in. and no more than 3/4 in. wide.