A straightedge or straight edge is a tool used for drawing straight lines, or checking their straightness. 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.
A pair of straightedges called winding sticks are used in woodworking to make warping easier to perceive in pieces of wood.
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