 On an upstream manifold the left and right cylinder valves allow the corresponding first stage regulator to be shut off, leaving the entire gas supply to be used through the remaining regulator.On an isolation manifold, the central valve, called the "isolating valve", separates the tanks into two independent systems, each with its own first-stage and second-stage regulators, which can prevent an upstream failure in one half of the system from losing the entire gas supply.Manifolded twin and triple cylinder sets have been used since the days of Cousteau and Gagnan's development of the open circuit regulator, as can be seen from early photographs of the equipment.