A straightedge is a tool with an edge free from curves, or straight, used for transcribing straight lines, or checking the straightness of lines. If it has equally spaced markings along its length it is usually called a ruler.
Straightedges are used in the automotive service and machining industry to check the flatness of machined mating surfaces.
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.
Read more about Straightedge: Compass-and-straightedge Construction