A tri square, also known as a try square, is a tool used in carpentry and metalwork. It consists of two parts; a steel blade and a wooden stock which also serves as the handle. These two parts are set at right angles to each other to form an L shape. A try square can be used to test that edges and corners have been cut straight. It can also be used as a guide for marking lines with a knife or pencil prior to cutting.
Hold the try square by the stock with the blade pointing forward and press it firmly against the item you wish to check so that the blade rests on the top edge.
Check to see if any light is visible between the stock, the blade and the item. If there is, the item is not square.
Lay the blade of the try square flat against the item you wish to cut. Push the stock up to the edge and hold it firmly to ensure you mark a true right angle.
Use a pencil or knife to mark a line on the item and follow this while cutting.
Try squares used in metalwork usually have metal rather than wooden stocks. Instead of a pencil, a scribing tool is used to scratch a line in the metal prior to cutting or shaping.