|In a stunning development, the lead picture for a post on miter joints is of a miter joint!|
"Oh my will he end the suspense and talk about the miter joints already."
Sure. Since you asked nicely.
Miter joints are angled cuts that, when put together, will form a closed shape. The most common miter joint is the 45 degree angle. Two 45 degree angles that come together will make a 90 degree corner, so four corners with 45 degree cuts gives you a closed square (4 x 90 degrees = 360 degrees). It's not rocket science (although with a little effort it could be trigonometry, damn you Mrs. Garibaldi and your prescient knowledge of what math would be useful in my life. If only you were a good teacher like Mr. Schimdt was...).
This is a miter saw, which can be called a compound miter saw, miter box, or chop saw:
|"I have so many names!:|
|0 degrees makes a square cut|
|45 degrees makes a 4 sided square frame|
|It clicks in at 45 degrees as well|
It is possible to miter shapes that have more than four sides too. The formula to figure out what angle is required for what number sided object is: (360 / number of sides)/2 = miter angle
360 / 6 = 60 / 2 = 30. 12 cuts at 30 degree miter cuts make up a hexagonal frame.
Here's the thing about miter joints: They are weak. You are dealing with end grain glued to end grain, which is not particularly ideal for glue adhesion. Furthermore, just picture the joint and the stresses on that joint. Two pieces of wood glued together on a miter don't have anything mechanically holding them together except for the glue. While wood glue is strong, it really is not that strong.
Miter joints are aesthetically pleasing. They make great picture frames, clock frames, and other framers. They make good trim joints. They make good things that don't need to be strong joints. The stresses on a picture frame are such that the inherent weakness of the miter joint does not outweigh the benefit gained from its aesthetic value.