What are miters? Miters are just angled cuts, when put together make a larger angle. The most common example is making a right angle, 90 degrees. It is made of two 45 degree cuts.
Why do we use them? We use miters for several reasons. It helps us keep consistent grain or to have a consistent pattern carry around a border, as in the below picture. If we just butted the frame parts together, the pattern would hit an abrupt stop. It also helps us hide the endgrain of wood by concealing it in the joint. It adds beauty and finesse to a piece, that leaves the onlooker guessing just for a moment.
Good Miters should be clean and well fitted the length of the joint. They should be paper thin. There should be no opening. Any putty or filler will be visible as a discoloration and is a sign of an poorly cut or clamped joint.
When Miters are made on a boards End Grain, an additional support is required for a strong joint. Just gluing End Grain to End Grain makes for weak construction as the End Grain will absorb some of the glue. The protrusions shown in Orange in the image to the left are added tenons, biscuits, or splines to increase the gluing area and add a mechanical element to the joint.
Exterior reinforcement can be used to strengthen a Miter as well. Splines are shown on the left in green. Channels are cut into the miter and the splines are then glued in and trimmed off. The result is a much stronger Miter with an added decorative element. The splines can be of a contrasting material or of the same material for a more subtle element.