of the set, but is not technically part of the breathing apparatus. The cylinder is usually worn on the back. "Twin sets" with two low capacity back-mounted cylinders connected by a high pressure manifold were more common in the 1960s than now for recreational diving, although larger capacity twin cylinders ("doubles") are commonly used by technical divers for increased dive duration and redundancy. At one time a firm called Submarine Products sold a sport air scuba set with three manifolded back-mounted cylinders. Cave and wreck penetration divers sometimes carry cylinders attached at their sides instead, allowing them to swim through more confined spaces. Newspapers and television news often wrongly describe open-circuit air scuba as "oxygen" equipment. Constant flow scuba Constant flow scuba sets do not have a demand regulator; the breathing gas flows at a constant rate, unless the diver switches it on and off by hand. They use more air than demand regulated scuba. There were attempts at designing and using these for diving and for industrial use before the Cousteau-type aqualung became commonly available circa 1950. Examples were Charles Condert dress in the US (as of 1831), "Ohgushi's Peerless Respirator" in Japan (a bite-controlled regulator, as of 1918), and Commandant le Prieur's hand-controlled regulator in France (as of 1926); see Timeline of diving technology. Open circuit demand scuba This system consists of one or more diving cylinders containing breathing gas at high pressure, typically , connected to a diving regulator. The demand regulator supplies the diver with as much gas as needed at the ambient pressure. This type of breathing set is sometimes called an aqualung. The word Aqua-Lung, which first appeared in the Cousteau-Gagnan patent, is a trademark, currently owned by Aqua Lung/La Spirotechnique. Twin-hose demand regulator This is the first type of diving demand valve to come into general use, and the one that can be seen in classic 1960s television scuba adventures, such as Sea Hunt. They were often use with manifolded twin cylinders. 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