Shut-off valves are used to control the flow of air or liquids. Air cocks control the flow of air into a plumbing system or gas line, while drain cocks allow the drain down of water. Water supply stops shut off the water supply to a faucet, toilet, or an appliance such as a washing machine. Water valve kits control the mixing of water, chemicals and inert gases. Check valves prevent backflow from contaminating clean water. Ball valve assemblies are for use with water, oil, and natural gas systems.