When two pieces of trim meet at an angle—most commonly a 45-degree angle to form a 90-degree corner—this is called a mitered joint. In the case of a typical window casement, miters are cut across the face of the moulding; for a return or scarf joint, the cut is across the thickness. Crown moldings require compound miters, which are cuts across both.
Outside corners of crown moulding, chair and picture rails, and baseboards require outside miters. Inside corners require coped joints. The corners of window and door casings require flat miters like the joints in picture frames.
For the mating pieces to fit together seamlessly, each must be cut at precisely the same angle. In cases where the angle may not exactly total 90 degrees, you can trim up the pieces after cutting according to the angle guides on your miter saw.
To cut any other size angles, divide the total angle of the corner in half, adjust your miter saw’s guide, cut, and then fit the pieces together. Trim or back-cut as needed to produce a tight joint.
After drilling pilot holes, nail in the pieces, and then place one “clinching” nail through an edge to help secure the joint. Keep nails about 3/4 inch from the end of a joint to prevent splitting the wood.
