A mortise and tenon is a trusted and commonly used furniture joint. Its used most often in a table or bed frame, where weight makes structure vital. The mortise is a rectangular or oval hole, that is cut out of the leg. The tenon is the shaped end of another piece of wood that fits into the mortised hole.
The mortise is usually placed in the leg and is cut into the face grain. The tenon is made by trimming the end of the apron or runner to fit snugly in the mortise.
This joint is often considered a sign of a higher quality table or bed. A well constructed mortise and tenon increases the stability of the furniture and when well executed and glued, the pieces are considered inseparable. It prevents table legs from racking and shifting over time, there are no bolts that can become loose, and will last as long as the wood has not rotted or become compromised. It is not intended to be disassembled so does not work well with knock down or flat pack furniture.
A simple and effective alternative to the Mortise and Tenon is the Floating Tenon. The mortise is cut into both of the components that are to be joined. Then an additional pieces is cut and glued into both mortises. While arguably not as strong as the mortise and tenon, it is very close in strength and will work for nearly all the same applications as the traditional.
Both the Floating Tenon and regular Mortise and Tenon can be reinforced mechanically. This would be accomplished by drilling holes through the leg and tenon while the tenon is inside. Then a wood or metal dowel is driven through both while assembled. This type of joint is not going anywhere.