Ball bearings are placed in wheels and allow them to roll. Typically, 7mm bearings are used on quad artistic skates, but 8mm bearings can be used on certain plates that have an 8mm axle. Most inline skates use 8mm bearings. The ABEC rating determines the tolerances in the bearing; the higher the ABEC rating, the more tolerance. Steel ball bearings are most commonly used, but many figure skaters may use ceramic bearings for better acceleration and smoother roll when tracing figures.