x_{1} is an open set in the topological space x_{2}.

x_{2} is conceptually composed of an underlying set, which is a superset of x_{1}, and topological information/defining information; however, it can also be expressed as a set of some of the subsets of the said underlying (implicitly define) set. Confer: "sezbarkle".

- sezbarkle
- x
_{1}(metric space/set/class/structure) is a metric subspace/subset(/subclass) of x_{2}(metric space/set/class) which is closed therein. x_{1}is an closed subset of x_{2}, where closedness is taken to be understood as being considered within x_{2}and under the metric shared by x_{1}and x_{2}