Covers are commonly used in the context of topology. If the set "X" is a topological space, then a "cover" "C" of "X" is a collection of subsets "U"α of "X" whose union is the whole space "X". In this case we say that "C" "covers" "X", or that the sets "cover" "X". Also, if "Y" is a subset of "X", then a "cover" of "Y" is a collection of subsets of "X" whose union contains "Y", i.e., "C" is a cover of "Y" if