Graduated ruler with perspective scales

Abstract From the Canadian patent

A straightedge having incremental markings along two opposite edges, the spacing of the markings being progressively decreased toward a theoretcial vanishing point to provide a perspective scale. The straightedge has a rigid spine with, two edge members of transparent material on which the graduations are marked. The edge members are offset toward opposite faces of the spine, so that when one member is held flat on a work surface, the other member is raised above the surface allowing, the raised edge to be used to guide or support a pen brush, or, the like without the ink or other fluid marking material running under the edge. The raised edge may be rocked into contact with the work surface …