Let be a regular parametrization of a surface in , where is a smooth vector-valued function of two variables. It is common to denote the partial derivatives of with respect to by , . Regularity of the parametrization means that and are linearly independent for any in the domain of , and hence span the tangent plane to at each point. Equivalently, the cross product is a nonzero vector normal to the surface. The parametrization thus defines a field of unit normal vectors :