A mitering box is a U-shaped carpentry box that is pre-grooved with a standard group of angles. A carpenter can place a piece of trim molding or other lumber into the box and with a hand saw they can select their chosen angle and cut the wood to that precise angle. For standard miter cuts, this type of simple process was common until the proliferation of mitering power tools. Today, a trim carpenter may use a portable mitering saw that has digital or mechanical degree indicators that allow the carpenter to make precision mitered cuts with minimal risk of lumber slippage during the cutting process.
Mitering is typically done by fine wood craftsmen such as trim carpenters, cabinetry carpenters, furniture makers, wood frame makers and other wood craftsmen that use trim moldings in their carpentry work.
Most home trim molding around doors and windows are mitered. Because home construction is not a precise environment, many visually straight walls and ceilings are not actually square. The carpenter measures to determine the real degrees of their mitered cut. This mitered angle may be several degrees off true or off square. The carpenter will create the illusion of a square fit around the window or door by expanding or contracting the miter degree or angle.
Mitering on cabinetry is usually more precise since the work is often shop produced where the level of control of square is greater. With cabinetry, the individual moldings will generally miter at 45 degrees for a finished corner of 90 degrees.
Homes with bay windows, octagonal rooms, vaulted ceilings and other unusual features press the carpenter to calculate the correct miter measurement to create the joinery effect of the lumber or molding being fitted to existing walls, cabinets or features. Some carpenters will also sculpt one side of off-cut pieces, particularly when compound mitering is necessary. A compound miter occurs when two angles are in play on each wood piece, such as with crown molding, that will corner on both the wall and the ceiling. A compound miter is the requirement of more than one miter occurring at a single joint on a single piece of lumber. This type of compound mitering is more advanced and can challenge a beginning carpenter.