Slip joints are essentially open-ended mortise-and-tenon joints, and resemble simple lap joints. Essentially, they are formed by creating a slot along the end of one board, the same width as the tenon of the other board which is formed normally.
The mortise slot is relatively easy to make on a tablesaw using a mortising jig, or even an appropriately-configured router. Traditional mortises require the use of a drill-press or hand-chisel. One disadvantage of this joint is that the cheeks must be clamped together during glueup, in addition to the regular clamping required to keep the mortise against the shoulder of the tenon.
Tapered Slip Joint
When applying a veneer to the face of a frame, the joint line where the grain changes direction may move at different rates, and may eventually cause damage that will be visible in the veneer. To prevent this, the tenon is cut so it is almost full-thickness at the shoulder, and slightly narrower than normal at its end. The slot is tapered to match the tenon. The purpose of this construction is to reduce the amount of wood movement at the transition point where the two grain-directions meet, and distribute the stress over a wider area. This is usually sufficient to prevent any visible damage to the veneer surface.
Mitered Slip Joint
A slip joint with the appearance of a mitered joint can be made by first mitering the slot of the slip joint, then by cutting the tenon on an angle to match the miter so the shoulders have a mitered appearance, but the tenon itself ends square. A blind mitered slip joint can be constructed by stopping the mortise before it becomes a complete slot, and trimming the tenon to match.
A bridle joint is formed by creating a groove on both sides of a board and inserting it into the slotted end of an open-ended slip joint, giving the appearance of a T-joint. This joint offers great load-carrying capacity, as the vertical member can support the weight of the crossing board. To hold maximum weight, the slot should be two-thirds of the board’s thickness. For maximum twisting strength, the slot should be one-half of the board’s thickness to make the (total of the) slip’s walls and the middle tenon the same thickness.