All the stages of this type of regulator are in a large valve assembly mounted directly to the cylinder valve or manifold, behind the diver's neck. Two large bore corrugated rubber breathing hoses connect the regulator with the mouthpiece, one for supply and one for exhaust. The exhaust hose is used to return the exhaled air to the regulator, to avoid pressure differences due to depth variation between the exhaust valve and final stage diaphragm, which would cause a free-flow of gas, or extra resistance to breathing, depending on the diver's orientation in the water. In modern single-hose sets this problem is avoided by moving the second-stage regulator to the diver's mouthpiece. The twin-hose regulators came with a mouthpiece as standard, but a full-face diving mask was an option.