Two open subsets (see open set) U and V of ℝⁿ are diffeomorphic if there is a diffeomorphism φ‎:U → V between them, that is a bijection such that both φ‎ and its inverse φ‎⁻¹ are differentiable. These ideas then generalize to manifolds and subspaces of manifolds. See also differential, transition map; compare homeomorphism.