A seal is used to make pressure-proof the interface (contacting surfaces) between two parts that have frequent or continual relative rotational or translational motion; such seals are known as dynamic seals, as compared with static seals. While the pressure in seals is lower than that in gaskets, the motion hinders their effectiveness so that there are more types of seals than gaskets, each type attempting to serve its environment. The materials are leather, rubber, cotton, and flax, and for piston rings, cast iron. The forms of nonmetallic seals are rectangular, V ring, and O ring. Cartridge seals are available for rolling-contact bearings. Special seals include carbon ring and labyrinth seals for turbines and mechanical seals for pumps.