A Tri Square is a tool often used by carpenters. It's generally made of steel and takes the shape of a right triangle (hence, the name) It will let you determine a 90 or 45 degree angle. This is handy when you need to make an accurate saw cut. You would just scribe along the appropriate edge of the tool. It often has inch marks along one or more of the sides for quick measuring. It may also have degree marks to help make cuts that are other than 90 or 45 degrees.