Hydrants are aboveground extensions of underground water mains, equipped with valves and/or connections for attaching fire hose. On FM Approved models, the pressure containing parts are usually made of cast iron or ductile iron. The valve seat, valve guide and other important working parts are usually made of bronze. A weather cap on the operating stem nut prevents the accumulation of ice, which would cause the hydrant to bind. Hose nipples are threaded, leaded and pinned, or twist-locked and set-screwed into the hydrant upper body.