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For the subculture, see Straight edge.
A straightedge is a tool with an accurately straight edge used for drawing or cutting straight lines, or checking the straightness of lines. If it has equally spaced markings along its length it is 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 is possible to do all compass and straightedge constructions without the straightedge. That is, it is possible, using only a compass, to find the intersection of two lines given two points on each, and to find the tangent points to circles. It is not, however, possible to do all constructions using only a straightedge. It is possible to do them with straightedge alone given one circle and its center. [edit] References
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