Multiple-spline joints are created by cutting slots in both pieces then gluing the joint together with separate splines inserted in the slots. Where the mortise-and-loose-tenon joint has a floating tenon that is oriented from edge to edge of the board, the multiple-spline joint has splines oriented from face-to-face, giving the appearance of box joints. And like box joints, multiple-spline joints can be easily formed on the tablesaw. The splines also offer plenty of long grain-to-long grain gluing surfaces to form a strong bond.
Many variations on the multiple-spline joint exist without changing its basic characteristics. The splines can be of virtually any length or thickness or can even be different lengths and thicknesses. The splines can be evenly spaced or they can be arranged in some form of pattern. The splines can be made of the same material as the boards being joined, or it can be made of a different, contrasting material. The thing to keep in mind is that the eye will search out a pattern, and if the arrangement does not reveal a pattern, the observer may have difficulty accepting the design.
If the spline is not visible from the front but is visible from the side, it is a half-blind joint. To form this joint, the front panel receives stopped grooves, similar to the creation of half-blind dovetail tails. The mating board receives the normal slotted treatment for multiple-splines.
One variation is to create the slots on the mating board less than full thickness, and make the slots on the front panel the same depth. This will create a multiple-spline joint that is visible from the side, but not visible from the front or from the inside.
Whereas the traditional miter joint is relatively weak due to its end grain-to-end grain gluing surface, inserting multiple splines into the joint greatly increases the long grain-to-long grain glunig surface which strengthens the joint. The full-blind multiple-spline joint has the appearance of a simple miter joint, but is strengthened and reinforced by the integral splines.
Though this joint appears difficult to manufacture, it is relatively simple with the use of a router and appropriate jig. It is cut similar to making dovetail tails on two panels, then inserting appropriately sized splines during the glue-up process. Accuracy is critical because the alignment of the slots on both boards must match.