Given topological spaces X and Y, the product space topology on the Cartesian product X×Y has the sets U×V, where U is open in X and V is open in Y, as a basis. The product topology on ℝ×ℝ is the same as the topology on ℝ2, though the basic open sets for the former might be considered as open rectangles and for the latter open discs.
If (X,dX) and (Y,dY) are metric spaces, then
is a metric on X×Y which induces the product topology.