Compass

A tool with two arms joined at their ends which can be used to draw circles. In geometric constructions, the classical Greek rules stipulate that the compass cannot be used to mark off distances, so it must "collapse" whenever one of its arms is removed from the page. This results in significant complication in the complexity of geometric constructions.

REFERENCES:

Dixon, R. "Compass Drawings." Ch. 1 in Mathographics. New York: Dover, pp. 1-78, 1991.