A straightedge's a tool with an accurately straight edge used for drawing or cutting straight lines, or checking the straightness of lines. If it's equally spaced markings along its length it's usually called a ruler. True straightness can in some cases be checked by using a laser line level as an optical straightedge: it can illuminate an accurately straight line on a flat surface such as the edge of a plank or shelf. It's possible to do all compass and straightedge constructions without the st… (More on Straightedge)