Plug valves are valves with cylindrical or conically tapered plugs, which can be rotated inside the valve body to control flow through the valve. The plugs in plug valves have one or more hollow passageways going sideways through the plug, so that fluid can flow through the plug when the valve is open. They are simple and often economical. When the plug is conically tapered, the stem/handle is typically attached to the larger diameter end of the plug. The simplest and most common general type of plug valve is a 2-port valve, which has two positions, open to allow flow, and shut (closed) to stop flow. Slightly conically tapered metal plug valves are often used as simple shut-off valves in household natural gas lines. Stopcocks used in laboratory glassware are typically forms of conically tapered plug valves. The plugs can be made of a similar plastic or glass.