Improper element

An element of a polytope is called improper whenever it's the minimal or maximal element of the polytope. These elements can be thought of as the empty set and the entire polytope. All other elements are said to be proper.

Traditionally, these elements aren't considered. However, taking them into account is mathematically convenient in a lot of circumstances. If we were to ignore them, the following annoyances would occur:
 * Stating the diamond condition would require us to special-case that edges have two vertices and ridges join two facets.
 * Convex polytopes would no longer form a lattice.
 * The nullitope and the point would no longer be distinct.
 * Polytope products would have a much messier formal definition.
 * The n-simplex would no longer have $$2^{n+1}$$ elements, instead having exactly two less.