Surface-supplied divers are generally required to carry an emergency gas supply, also known as a bailout set, which is usually back-mounted open circuit scuba connected into the breathing gas supply system by connecting an interstage hose to the gas switching block, (or bailout block), mounted on the side of the helmet or full-face mask, or on the diver's harness where it can easily be reached, but is unlikely to be accidentally opened. Other mounting arrangements may be used for special circumstances.