Most rolling bearings consist of an inner ring and an outer ring, rolling elements (either balls or rollers) and a retainer (cage). The cage separates the rolling elements at regular intervals holds them in place within the inner and outer raceways, and allows them to rotate freely.  Rolling elements come in two basic shapes: ball or rollers. Rollers come in four basic types: cylindrical, needle, tapered, and spherical. Balls geometrically contact the raceway surfaces of the inner and outer rings at points, while the contact surface of rollers is a line contact. Theoretically, rolling bearings are so constructed as to allow the rolling elements to rotate orbitally while also rotating on their own axes at the same time. While the rolling elements and the bearing rings take any load applied to the bearings (at the contact point between the rolling elements and raceway surfaces), the case takes no direct load. It only serves to hold the rolling element at equal distances from each other and prevent them from falling out.