diving operation. 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. Side-mount harnesses support the cylinders by clipping them to D-rings at chest and hip on either or both sides, and the cylinders hang roughly parallel to the diver's torso when underwater. The harness usually includes a buoyancy compensator bladder. It is possible for a skilled diver to carry up to 3 cylinders on each side with this system. An unusual configuration which does not appear to have become popular is the integrated harness and storage container. These units comprise a bag which contains the buoyancy bladder and the cylinder, with a harness and regulator components which are stored in the bag and unfolded to the working position when the bag is unzipped. Some military rebreathers such as the Interspiro DCSC also store the breathing hoses inside the housing when not in use. Technical divers may need to carry several different gas mixtures. These are intended to be used at different stages of the planned dive profile, and for safety reasons it is necessary for the diver to be able to check which gas is in use at any given depth and time, and to open and close the supply valves when required, so the gases are generally carried in fully self-contained independent scuba sets, which are suspended from the harness at the diver's sides. This arrangement is known as stage mounting. Stage sets may be cached along a penetration guideline to be retrieved during exit for convenience. These are also sometimes called drop tanks. Harness construction Every scuba harness requires a system for supporting the cylinders on the harness, and a system for attaching the harness to the diver. Basic harness The most basic arrangement for a back-mounted set consists of a metal or webbing strap around the cylinder just below the shoulder, and another lower down the cylinder, to which webbing shoulder and waist straps are attached. Shoulder straps can be of fixed length to suit