The Presta valve is a valve (commonly) foun in high pressure road style and higher end mountain bicycle tires, although it is now used on all UST or "tubeless" mountain bike tires. It consists of three major components: the valve body, a threaded valve stem, and a lock nut. In order to accommodate different rim wall depths, the valve body is available in various lengths. Like the Schrader valve, air pressure inside the tire holds the valve shut; however, unlike the Schrader, the Presta has no return spring to help keep the valve shut. Instead, a threaded stem extends out of the valve core to hold a locking nut that can be screwed down to ensure the valve remains closed. A Schrader core can be unscrewed and replaced; some Presta valves, too, have removable cores.