A scuba set, originally just scuba, is any breathing apparatus that is entirely carried by an underwater diver and provides the diver with breathing gas at the ambient pressure. Scuba is an anacronym for self-contained underwater breathing apparatus. Although strictly speaking the scuba set is only the diving equipment that is required for providing breathing gas to the diver, general usage includes the harness by which it is carried, and those accessories which are integral parts of the harness and breathing apparatus assembly, such as a jacket or wing style buoyancy compensator and instruments mounted in a combined housing with the pressure gauge, and in the looser sense, it has been used to refer to all the diving equipment used by the scuba diver, though this would more commonly and accurately be termed scuba equipment or scuba gear. Scuba is overwhelmingly the most common underwater breathing system used by recreational divers and is also used in professional diving when it provides advantages, usually of mobility and range, over surface supplied diving systems, and is allowed by the relevant legislation and code of practice. Two basic functional variations of scuba are in general use: open-circuit-demand, and rebreather. In open-circuit demand scuba, the diver expels exhaled breathing gas to the environment, and each breath is delivered at ambient pressure, on demand, by a diving regulator, which reduces the pressure from the storage cylinder. The breathing gas is supplied through a demand valve when the diver reduces the pressure in the demand valve housing during inhalation. In rebreather scuba, the system recycles the exhaled gas, removes carbon dioxide, and compensates for the used oxygen before the diver is supplied with gas from the breathing circuit. The amount of gas lost from the circuit during each breathing cycle depends on the design of the rebreather and depth change during the breathing cycle. Gas in the breathing circuit is at ambient pressure, and stored gas is provided through regulators or injectors, depending on the design. Within these systems, various mounting configurations may be used to carry the scuba set, depending on application and preference. These include: back mount, which is generally used for recreational scuba and for bailout sets for surface supplied diving; side-mount, which is popular for tight cave penetrations; sling mount, used for stage-drop sets; decompression gas and bailout sets where the main gas supply is back-mounted; and various non-standard carry systems for