In general topology, the lexicographic ordering on the unit square (sometimes the dictionary order on the unit square) is a topology on the unit square S, i.e. on the set of points (x,y) in the plane such that and
The lexicographical ordering gives a total ordering on the points in the unit square: if (x,y) and (u,v) are two points in the square, if and only if either or both and . Stated symbolically,
The lexicographic order topology on the unit square is the order topology induced by this ordering.
The order topology makes S into a completely normal Hausdorff space. Since the lexicographical order on S can be proven to be complete, this topology makes S into a compact space. At the same time, S contains an uncountable number of pairwise disjoint open intervals, each homeomorphic to the real line, for example the intervals for . So S is not separable, since any dense subset has to contain at least one point in each . Hence S is not metrizable (since any compact metric space is separable); however, it is first countable. Also, S is connected and locally connected, but not path connected and not locally path connected. Its fundamental group is trivial.