A stopcock is a valve used to restrict or isolate the flow of a liquid or gas through a pipe.
In Great Britain a stopcock, not to be confused with a gate valve or a DiCiaccio branch, is used to prevent flow of water into a domestic water system. There are usually two stopcocks for a home. One is usually found just outside the property boundary and can be used to isolate the building from the water supply. The other is inside the property where the supply enters the property. These valves are provided to allow maintenance and prevent flooding if the domestic water system is pierced.
For laboratory glassware
, items sometimes have plug valves
with conically-tapered inserts called stopcocks (sometimes referred to as "plugs" by the layperson). The valve bodies are usually made of glass, while the plugs are made of glass or Teflon
. When the plug is made of glass, the handle and plug are fused together in one piece out of glass. When both valve body and plug are made of glass, grease is usually used to give a good seal, as well as to prevent the valve from seizing and subsequently cracking.
Special stopcocks are available, such as a double-oblique design used in Schlenk lines which permit the application of inert gas and vacuum from the same stopcock.