Many woodworkers find it difficult to believe, however the simple glued edge joint is remarkably strong. Having a good clean and flat edge mating with another similar flat edge produces a joint that is just as strong as the surrounding wood.
The use of joining devices such as splines, biscuits, dowels or keys can actually weaken some joints especially when the material of the device is weaker than the wood to which it is attached. Though they do not impart any strength to the joint, these devices do offer one important advantage: They give the woodworker a handy alignment device that can speed assembly, or maintain alignment during the gluing and clamping process.
Biscuited joints use a series of small slots cut into the edges of both boards to be joined, into which small oval or disk shaped piece of wood or other fibre which are placed. The biscuits maintain an almost perfect surface alignment between the two pieces of wood, yet allow for end-to-end movement between the two pieces. This end-to-end movement helps to make up for slightly mis-aligned slots in adjoining pieces.
Use the largest biscuit that will fit the slot, since a loose biscuit will allow the two wood pieces to drift. It is the stiffness of the biscuit in the slot that gives the alignment assistance that you are looking for.
This is a variation of a biscuit joint. Instead of using numerous smaller biscuits, splines are essentially a single long biscuit used to align the two pieces. The spline is a piece of plywood or hardboard that is placed in a slots that are cut in the adjoining edges. These slots can be stopped so they do not show if the ends are to be exposed. One thing to keep in mind is to ensure the spline is slightly narrower than the depth of the slot. Making the spline exactly the depth of the slot can lead to splitting of the wood as the surrounding wood shrinks, but the hardboard spline does not (hardboard does not shrink in the same manner as hardwood). A gap of 1/32 (1/64th on either slot) is sufficient to prevent this problem.
Dowelled edge-to-edge joints are another variation on the biscuit joint, but use dowels instead of biscuits. The dowelled joint is prone to errors associated with the difficulty in drilling perfectly perpendicular holes in the narrow edge of panels. This problem can be managed through the use of specialty drilling rigs that drill the alignment dowlel holes in a regular pattern, but these machines are beyond the reach of most hobyists, and other simpler techniques produce similar or better results. As with splines, ensure that the holes drilled for dowels are slightly deeper than the dowel is long.
Butterfly keys are a traditional japanese jointing technique, and it provides a very decorative way to edge-join boards. It adds visual appeal to the joint, and depending on the size of the key, the key can be used without adding glue to the joint which offers the possibility of disassembling the boards.
The keys themselves can be made of wood, or even from certain metals such as brass. Note that certain metals are not suitable for use because they will rust in the presence of the natural moisture in the wood.