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.