A mitre box (US spelling, "miter box") is a woodworking tool used to guide a hand saw to make precise mitre cuts in a board.
The most common form of a mitre box is a 3-sided box which is open at the top and the ends. The box is made wide enough to accommodate the width of the boards to be cut. Slots are cut in the walls of the box at the precise angle at which the cut is to be made. These slots provide the guide for the saw to follow. Most commonly the slots in the mitre box are cut at 45 degrees and 90 degrees. In use, the board to be cut is placed in the box and the point at which the board is to be cut is lined up with the appropriate slot in the mitre box wall. A tenon saw is the most common saw used for this operation. The cut is started by placing the saw blade in the slot and drawing the saw back and forth. The slot guides the cut all the way through so that the result is a cut that is perpendicular to the face or edge of the board (depending upon the desired orientation of the cut) and at the appropriate angle. For precise work, a board cut in a mitre box might then be further adjusted on a shooting board. In the past, mitre boxes were constructed mostly of wood. Some have had metal guides fitted to help prevent wear on the slots. Today, mitre boxes are also available in other materials such as moulded plastic and cast aluminium. When carpenters used to make their own mitre boxes it was common to use one for a while and then when the guide cuts got worn they were thrown away and a new one made very quickly out of scrap timber. The three pieces of timber are nailed together, and the top steadying pieces are fixed with smaller panel pins. The angles are marked, (any angle, not necessarily 45 deg), and the guide cuts are cut with the same saw that will be used for the rest of the cutting. There is still a case to be made for making one today. For example if an odd size, say 175mm cornice (crown moulding) is to be fixed, Then a bottom board 175mm wide would make sure that the cornice sits true while the cuts are being made. And there you go. The mitre saw combines the features of a mitre box with a tenon saw.
