Multiple-Tenon Case Joints
Mortise-and-tenon joints are usually considered frame joints, but they serve an important purpose in case construction as well.
Multiple-tenon joints for case construction are primarily used for center supports, and seldom, if ever, for the edge of the case. Because of this, the joint has two primary functions: to support the load that might be placed on the board, and to lock the board in place to prevent it from moving. At first look, a multiple-tenon joints resemble blind stopped dado joints, however the multiple-tenons offer some advantages. The first is that board with the mortises is not weakened from having a full-length dado cut through it as in the dado joint. The second is that the multiple tenons add more gluing surface area. And finally multiple-tenon joints provide positive registration for board placement to keep the board from shifting. A convenient side-effect of that increased gluing surface area is that the joint can be used in situations when it needs to hold the case sides together instead of just resting in position.
The tenons can be made either blind (not visible from the surface of the joint), or through (where the tenons are visible from the exterior of the joint). Through tenons are traditionally secured in place using a small wedge that is inserted into a cut that is made into the end grain of each tenon. This wedge can even be made with a contrasting wood to add visual detail, or it can be made from the same wood so as to not draw attention to the joint.
The tenons of multiple-tenon joints are customarily the full thickness of the stock, providing incredible strength when used for shelving. As the board becomes wider, more tenons should be used to provide more distributed support. In this respect, multiple-tenon joints are similar to box joints except they are located in the center of boards instead of the end of boards.
Twin-Tenon Case Joints
One variation of the multiple-tenon case joint is to use a pair of mortises and tenons. Visually, this gives the joint the appearance of a multiple-tenon joint being constructed on a blind dado, and it gives some of the same benefits. When the board is placed in the mortises, the load rests on the dado portion, while the tenons hold the board in place and prevent it from being pulled out of the joint.