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.