» For the subculture, see Straight edge. A straightedge is a tool similar to a ruler, but without markings. Compass-and-straightedge construction An idealized straightedge is used in compass-and-straightedge constructions in plane geometry. It may be used: Given two points, to draw the line connecting them. Given a point and a circle, to draw either tangent. Given two circles, to draw any of their common tangents. It may not be marked or used together with the compass so as to transfer the… (More on Straightedge)