Mechanically transferring the observed angle of a receeding edge to the drawing surface requires that the tool be parallel to the picture plane because only then will it be easily transferred to the drawing surface. When the tool is perpendicular to the line of sight it will be, for all practical purposes, parallel to the picture plane . If we tilt the straight edge toward the objects so that it mimics the actual direction of the receding edge, we undermine the overall effectiveness of the “clock angle tool.” This should be avoided because tilting the straight edge it positions the straight edge oblique to the picture plane thereby making it very difficult to transfer the observed angle to the drawing surface.
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