Compass and straightedge drawing is used to show things in plane geometry (geometry used on a flat area). Real compasses are used on paper, but the perfect compass used in proofs is an imaginary, perfect tool that draws perfect circles. The most strict definition of this ideal tool, used for explaining, is the "collapsing compass". It draws a circle from a given place with a given radius and cannot be used again. That is, unlike a real pair of compasses, it cannot simply be moved to another place and used to draw another circle that is the same.