Miter joints are beautiful to behold when properly constructed. There is no visible end grain, and the seam where the two boards are joined produces a gentle change in grain direction that can be very pleasing.
Miter joints have several disadvantages to consider, some of which can be countered by using an appropriate option. The joint is essentially end grain to end grain which produces a relatively weak joint to begin with. Fasteners are an option however they must be driven into end grain, and the wood ends get progressively thinner which weakens the wood being held. And finally the assembly process can be irritating since clamping miter joints only seems to cause the boards to slide against one another instead of holding in place for the joint to set.
All of these disadvantages, however, can be mitigated by the use of an appropriate fastener to provide the structural strength.
Nails can be driven from both ends through the side of one board and into the end grain of the other board. Nailing into both directions prevents the boards from simply being pulled apart, and using two nails from each direction, each at skewed angles, further reinforces the joint. This option should only be used when joint strength is not critical.
Dowels can be used to align the two boards being joined. The holes can easily be drilled and aligned by the use of an appropriate jig, however the difficulty is in producing an accurately angled hole that aligns with its mate. Any mismatch in the alignment or angle of the holes will result in a joint that will not close.
Biscuits can be used as a floating tenon to join a flat mitered frame joint. These biscuits can help in the alignment of the joint during assembly, and produce a relatively strong joint since the long grain of the boards is glued to the neutral grain of the biscuit.
An alternative to a biscuit is to use a spline. It is a relatively simple matter to cut the slots into the ends of the two boards, then inserting the spline during the glueup process. If the spline is full-length, the spline can be cut to length after the glue has dried.
If the spline slot is stopped before it reaches the board's edge, you can create a full-blind or half-blind spline to hide the spline inside the joint. This gives you the strength of the spline without the visual interuption that the spline end would show. If you do decide to use a blind spline, cut the spline slightly shorter than the slot length to allow for some adjustement and fine-tuning during glue-up.
For maximum strength, the grain of the spline (if it has any) must run perpendicular to the miter. If the spline grain is oriented in the same direction as the joint cut, the spline will be prone to split along its full length. If hardboard or plywood is used, this is not a concern since they are considered grain-neutral.
In order to produce a joint where four different boards meet at a point, each board would first receive two 45° miter cuts to form a point on one end, then the tip would receive a slot that would be as deep as the point, but would not reach the board's edges. During glueup, a single square spline (slightly undersized!) would be inserted into the slots and the four points joined together. Then it is simply a matter of gluing the joint and clamping the boards with minimal force to prevent shifting.
In this joint a spline is inserted diagonally from edge to edge across the joint. Though the spline slot can be cut and the spline inserted before glue-up, it is far simpler to glue a simple flat miter joint with glue only, then once the glue has set you can cut the slot for the spline and insert the oversized spline key for a second glue-up. Once the glue has set, you can trim the spline to the appropriate size. This joint produces an abundance of long grain to long grain gluing surface, which produces a very strong joint.
If the spline material has a grain, then the grain should be aligned perpendicular to the miter, running from outer edge to outer edge. This will minimize any effect produced by wood movement.
As with most splines, the spline material can be of a matching or contrasting colour, depending on the effect you desire.
An alternative to using a single spline key is to use multiple feather keys. These keys are essentially made of thin veneer and are inserted into the very narrow kerf of specialty handsaw blades.
A dovetail key adds mechanical interlocking to the concept of a simple spline key or feather key. The wedge shape of the dovetail key prevents the joint from being separated, and the gluing surfaces presented are all long grain to long grain.
This joint is easiest to make by first producing a simple glued miter joint, then cutting the slot for the key with a router and an appropriate dovetail bit, or cutting the slot by hand. Once the slot is cut, you can then proceed to cut an appropriately sized angled wedge. You should make the wedge longer than you need so that you can trim the ends after assembly. Once you have made the dovetail key, simply insert the wedge with plenty of glue and clamp the joint until dry.
The limiting factor for the size of the key is the width and depth of the miter corner. The dovetail key must leave enough continuous wood to support the two slot walls, and therefore the width of the slot should not be more than two thirds of the frame's thickness, or one half of the frame's thickness if the frame is made from a soft wood. The angle of the key's sides should conform to the dovetail standards of 8° to 12°, since this provides the most strength without removing excessive material which would weaken the joint.
As with the spline key, the grain of the dovetail should run perpendicular to the miter cut.
The beauty of this joint is it leaves a dovetail detail visible from either end of the joint, and this can be highlighted by the use of contrasting coloured material for the key.
The butterfly key is an hourglass-shaped key that is inserted into the face material of the joint, spanning the two boards. The shape of the key prevents the two boards from being separated, and adds a decorative visual detail to the joint, especially if the key is made of a contrasting material.
The slot for the key is cut after glueup, using a router and/or a chisel. The narrow portion of the key should be perfectly aligned along the joint line. After the slot has been cut, a key can be cut to the exact shape then glued into the slot before being trimmed flush with the surface.
As with the spline key, the grain of the butterfly should run perpendicular to the miter cut, from wide portion to wide portion.