What is a Miter Joint?

A miter joint is a type of joint where two objects are joined at a beveled edge, often being an angle of about 90 degrees. The two pieces of material being joined typically are cut at 45-degree angles, so that when joined together, they form an angle of 90 degrees. Most miter joints are perpendicular, but there are some circumstances in which other measurements are used. Miter joints are most commonly used in picture frames or crown moldings and are even used to compact the larger pipes found on a pipe organ.

Sometimes, a miter joint is not perfectly perpendicular. Instead, some special circumstances require for different angles, such as installing trim on a house or creating an oddly shaped picture frame or box. Regardless of the application, the angles at which the material is cut must be measured precisely. Accurate and well-calculated measurements will allow for the two joined pieces to be flush with one another, which allow for strong, tight miter joints. The angle at which the pieces need to be cut is found by dividing the desired angle of the miter joint in half.

Miter joints do not exclusively consist of wood, nor are the materials used always flat. Three-dimensional miter joints can be found in many places, usually taking the form of piping arrangements. The curves and bends of piping in a plumbing application typically are formed through pipe couplings, but other types of piping employ mitered joints. On a pipe organ, the pipes — often known as either flues or reeds — can reach incredible lengths, with some larger organs having pipes up to 64 feet (19.5 m) long. The size of such of a pipe often is too large for most churches or concert halls, so a miter joint might be employed to reduce the size of the pipe.

Another type of miter joint is known as the compound miter joint. A completed border made with a miter joint is often perfectly square or rectangular, such as in a picture frame. Compound miter joints are different, because they have a different a number of sides and angles than what one would find on a box or frame. For this reason, the angles being cut must be calculated specifically according to the number of sides being used.

Adding strength to miter joints is very important, because they typically are relatively weak without re-enforcement. To help re-enforce a miter joint, a variety of methods can be used. These methods include things such as placing a block inside the joint or adding splines and dovetail keys. For wood miter joints, the lock miter joint is known to be relatively strong and successful. This type of joint is created through the use of a router, where the two pieces are joined together through cut notches.