If you are a DIYer you should buy this inexpensive gadget. The speed square consists of a triangle that has a lip (also called a fence) on one of its edges. It is a tool that allows you to create a straight line on a board that you can use as a guide. In addition, you can use it directly with a circular saw to make a straight cut in the field. However a speed square can do so much more. See the pictures below for more details.