The Presta valve (also called Sclaverand valve or French valve) is a valve commonly found in high pressure road style and some mountain bicycle inner tubes. It comprises an outer valve stem and an inner valve body. A lock nut to secure the stem at the wheel rim and a valve cap may also be present.
The outer valve stem is manufactured in various lengths for different applications, and has a narrower diameter (6 mm) than the Schrader valve (8 mm)). The weakest point of a bicycle rim is usually the hole for the valve stem. The smaller hole for a Presta valve makes it possible to have extremely narrow wheels while maintaining sufficient strength in the wheel. 
The air pressure in an inflated tire holds the inner valve body shut. A small screw and captive nut on the top of the valve body permits the valve to be screwed shut and ensure that it remains tightly closed. The nut must be unscrewed to permit airflow in either direction. This must be done before attaching a pump. The screw remains captive on the valve body even when unscrewed fully; it is tightened again after the tire is inflated and the pump removed. The valve cap protects the valve body, keeps dirt and mud out of the mechanism, and also prevents the valve from damaging the tube when it is rolled for storage, but is not necessary to prevent pressure loss.
Because the rims of bicycles drilled for Presta valves cannot accommodate the wider Schrader valves, it is often the case that rims need to be drilled for such replacements. Conversely, when a Presta valve is fitted into the larger Schrader rim hole, grommets or reducers are sometimes used to take up the extra space.
The standard Presta valve has an external thread. An adaptor can be fitted onto this external thread to permit the Presta valve to be connected to a pump with a Schrader chuck. The same adaptor, because of a coincidence of thread sizes, can convert a direct-fitting Schrader pump into one that can connect to flexible adaptors of either kind.
Unlike Schrader cores, the inner parts of the valve, the valve core, of standard Presta valves often cannot be removed from the stem. Since the core usually has to be removed to use a tubeless or Universal System Tubeless (UST) setup so that tire sealant may be added, removable core Presta valves have become more common.