Most recreational divers rely on their buddy for bailout gas, and do not carry a redundant gas supply. The position of the cylinder valve behind the head has proven to be reasonably safe in millions of dives, though some divers do have physical difficulty reaching the valve while wearing the set, particularly if the cylinder is mounted relatively low on the harness. Though sidemount divers may benefit from being easily able to see and manipulate valves, first stages or cylinders, this is rarely required in recreational use. In single cylinder diving there is seldom a reason to shut a cylinder valve while diving, and there is no need for changing cylinders or managing different gases. The recreational diver with a single cylinder is not supposed to enter low overhead spaces, so the single valve behind the diver's head is unlikely to come into contact with objects which might roll it closed.