End miter joints, in its most basic form, are formed by mitering the ends of two boards at a 45º angle, then butting the ends together. The disadvantage of this joint is that the glue is entirely on end grain (the weakest glue bond) and there is no form of interlocking between the boards. Basic miter joints are also notoriously difficult to clamp effectively, since the boards will always have a tendancy to shift against one-another.
The easiest way to strengthen an end miter is to drive nails or screws into each side of the joint, crossing directions to lock the joint together. Drilling pilot holes will help prevent splitting, and keeping the holes slightly towards the inside of the joint will reduce any damage to the face of the board should the joint fail and require repair.
Biscuits are small oval discs that are inserted into matching slots that are cut into the two boards to be joined. Choose a biscuit size that fits snugly into the slot, since any play will reduce the accuracy of the joint. Biscuits also provide positive registration for the matching boards, speeding the assembly and glue-up process. Just remember to keep the biscuit slots slightly towards the inside edge of the boards, to avoid weakening the exposed edge of the joint.
A spline is essentially a full-length biscuit. The slot is cut along the entire length of the joint on both boards, and a hardwood or hardboard spline is inserted during the glue-up process. The spline also provides positive registration which will help assembly, just like a biscuited joint, however a splined end miter joint suffers from a weakened outer corner since the wood at the corner is only connected by a thin strip once the spline has been cut. Because of this, it is essential that the slot for the spline be offset towards the inner corner of the joint.
This style of end miter joint is formed by creating a standard end miter joint, then cutting slots through the outside corner of the joint and gluing blocks or splines into the slots. Once the glue has set, the ends of the splines are trimmed flush with the ends of the joints. This has the advantage of adding strength to the joint, while giving it some visual appeal. Using a contrasting colour of wood gives the most dramatic effect. This joint can appear like a box joint, however the slots do not alternate on each side of the joint as the box joint would.
Using thin strips of wood instead of blocks gives the keys a more dellicate appearance. Restricting the slot width to the blade kerf makes this variation easy to make. The slots can also be easily made on any angle to increase the visual appeal, and will also add to the mechanical interlocking of the joint. The more keys that are used, the stronger the joint will become.
Instead of cutting the slots using a straight blade or bit, use a dovetail bit instead. This will produce a triangular shaped slot, into which you can insert a triangular shaped key which is then trimmed flush with the joint sides. This gives the visual effect of a dovetail joint on both sides of the joint. This variation also gives the joint some mechanical interlocking since the triangular keys prevent the joint from separating.