A: Compass with a sliding angle bar that revolves through 160 degrees on a pin at the back that can be secured at any angle by means of a screw. Affixed to the straight side of the compass is a superstructure that holds a rack bar. B: The rack bar consists of a steel bar with a brass rack screwed to the surface. C: Wooden box with three hinges on one side and a latch on the front. The box interior is lined with black velvet on the bottom.