Symmetry, in geometry, is a correspondence, or matching, of parts of an object. These parts correspond in size, shape, and position after certain geometric operations are carried out. One major use of the concept of symmetry is to classify crystals (see Crystal). In this application, three kinds of symmetry are especially useful. The operations that produce these symmetries occur relative to (1) a plane of symmetry, (2) an axis of symmetry, and (3) a center of symmetry.