Miter joint

Miter joint (pieces ready to be joined)
Miter joint of two pipes

A miter joint (mitre in British English), sometimes shortened to miter, is a joint made by beveling each of two parts to be joined, usually at a 45° angle, to form a corner, usually a 90° angle.[1] For woodworking, a disadvantage of a miter joint is its weakness, but it can be strengthened with a spline.[2] There are two common variations of a splined miter joint, one where the spline is long and runs the length of the mating surfaces and another where the spline is perpendicular to the joined edges.

Common applications include picture frames, pipes (e.g. pipe organs), and molding.

Non-perpendicular joints

For miter joints occurring at angles other than 90°, for materials of the same cross-section the proper cut angle must be determined so that the two pieces to be joined meet flush (i.e. one piece's mitered end is not longer than the adjoining piece). To find the cut angle divide the angle at which the two pieces meet by two. Technically two different cut angles are required; one for each piece, where the second angle is 90° plus the aforementioned cut angle, but due to angular limitations in common cutting implements (hand circular saws, table saws) a single angle is required and is used to cut the first piece in one direction and the second piece in the opposite direction.

3D joints

When a piece is beveled at both ends, such that the two attached pieces do not lie in the same plane, a three-dimensional structure is obtained. In that case, it is also necessary to either rotate the piece in its longitudinal axis or to tilt the saw blade before beveling the second end.

When employing the miter joint to connect two pieces that have a non-circular cross-section it is typically desirable to have the longitudinal edges of the joined pieces match up properly at the joint. It is always possible to close a [3]structure constructed with pieces having non-circular cross section into a loop through properly matched miter joints (e.g. a picture frame). However, a three-dimensional loop from pieces with non-circular cross section need not close properly when attempting to miter it all the way round. In general, a twist occurs, causing the edges at the last joint to be misaligned.