For topologoical spaces (Τ_{1},X_{1}) and (Τ_{2},X_{2}),
{(G_{1 }x G_{2})| G_{1} ∈ Τ_{1}; G_{2} ∈ Τ_{2 }is a base for the product topology T_{(X1 x X2) }We know:
B is a base for Τ iff
(1) B⊆Τ and,
(2) For every G∈Τ, if p∈G, then there is B∈B such that p∈B⊆G.