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A valve stem is a self-contained valve which opens to admit gas to a chamber (such as air to inflate a tire), and is then automatically closed and kept sealed by the pressure in the chamber, or a spring, or both, to prevent the gas from escaping. They are most commonly used on automobile and bicycle wheels, but also for many other applications.
Schrader valves (also called American valves) consist of a valve stem into which a poppet valve is threaded with a spring attached. They are used on virtually all automobile tires, and in the United States, on most wider rimmed bicycle tires. In addition to tires, Schrader valves of varying diameters are used in refrigeration and air conditioning systems, plumbing, fuel injection engines, suspension systems, and SCUBA regulators, allowing the user to remove and attach the hose while in use.
Presta valves (also called Sclaverand valves or French valves) are normally only used on bicycles. The stem has a narrower diameter (nominally 6 mm) than the thinnest (nominally 8 mm) Schrader type, and so the bore of the hole in the rim through which the stem passes can be smaller.
Dunlop (or Woods)Edit
Dunlop valves, (also called Woods valves or English valves) were once popular in Great Britain. They are still widely used on bicycle tires in many countries (Japan  the Netherlands, Germany, Third World), especially on low to medium priced bicycles.
Their bases are wider than Presta valves, but they can be pumped with Presta pumps. Pumps with a pincher-type connection however are the most common type used: the classic black pumps mounted on a wooden base. The classic pump does not allow for high pressures, 3.5 bar (50 psi) max.
Due to the construction, the Dunlop valve does not check the pressure. The actual valve can be replaced easily, without the need for special tools as with Schrader valves. This also allows use of a wide stem to fill the tube with a sealant that cures leaks while cycling.
Many other valves exist that are used only in certain regions or for limited purposes.
For example the "Regina Valve" is a valve very much similar to the Presta and mostly being used in Italy. Invisible tire valves are different in design from traditional tire valves. An inflator stem is not used; instead, a removable cap on a valve stem embedded in the tire rim is fitted, with only the cap visible. When inflating the tire, the cap is first removed, typically with a coin, and then a "portable" dedicated inflator stem is screwed onto the valve stem. Through the inflator the tire is inflated as usual.