A connected surface which is a 1-dimensional complex manifold, such as the Riemann sphere or the complex plane. Riemann surfaces can be thought of as deformations of the complex plane in the sense that the local topology can be that of the complex plane but the global topology may be quite different. Riemann surfaces are necessarily orientable. They arise naturally in the study of algebraic curves in the complex projective plane (see degree-genus formula, projective space); and closed, orientable surfaces can be given the structure of a Riemann surface (see uniformization theorem).