In general, one refers to the family of sets containing 0, used to approximate 0, as a ; a member of this neighborhood basis is referred to as an open set. In fact, one may generalize these notions to an arbitrary set ("X"); rather than just the real numbers. In this case, given a point ("x") of that set, one may define a collection of sets "around" (that is, containing) "x", used to approximate "x". Of course, this collection would have to satisfy certain properties (known as axioms) for otherwise we may not have a well-defined method to measure distance. For example, every point in "X" should approximate "x" to degree of accuracy. Thus "X" should be in this family. Once we begin to define "smaller" sets containing "x", we tend to approximate "x" to a greater degree of accuracy. Bearing this in mind, one may define the remaining axioms that the family of sets about "x" is required to satisfy.