A cross-capped disk is formed by identifying these pairs of points, making them equivalent to each other. This means that a point with parameters ("u", 1) and coordinates formula_4 is identified with the point ("u" + π, 1) whose coordinates are formula_5. But this means that pairs of opposite points on the rim of the (equivalent) ordinary disk are identified with each other; this is how a real projective plane is formed out of a disk. Therefore the surface shown in Figure 1 (cross-cap with disk) is topologically equivalent to the real projective plane "RP"2.