It is the evolution of a type of compass of ancient use, perfected in the beginning of the 17th Century by the Swiss inventor of instruments Jost BÅrgi. It is furnished with two arms, each one with a central slit in which a mobile pivot slides. The points are inserted in both of the extremities of the arms (for a total of four points). Sliding the central pivot, one can enlarge two of the four points in a determined relationship chosen on the basis of the indications reproduced along the arm and selected on the basis of the position of the pivot. Taking advantage of this relationship one can reproduce the points of which the distance is measured in scale.