Every simply connected Riemann surface is conformally equivalent (or biholomorphic) to precisely one of the Riemann sphere, the complex plane, or the open unit disc in ℂ. Consequently all Riemann surfaces that are homeomorphic to the sphere are, in fact, biholomorphic; by contrast, there are uncountably many Riemann surfaces that are homeomorphic to the torus but not biholomorphic. See also Classification Theorem for Surfaces.