By the duality principle, for every polyhedron, there exists another polyhedron in which faces and polyhedron vertices occupy complementary locations. This polyhedron is known as the dual, or reciprocal. The process of taking the dual is also called reciprocation, or polar reciprocation. Brückner was among the first to give a precise definition of duality. Starting with any given polyhedron, the dual of its dual is the original polyhedron.