 Multiple-tank configurations include downstream manifolded twins, with a single regulator, "independent" or "separate doubles" which are two cylinders clamped to a backplate, but without a manifold, side mount cylinders, or upstream manifolded twins, with two complete regulator sets, which may have an isolation valve.The function of a manifold is to connect the gas supplies of two back mounted cylinders (called "doubles" or "twins"), allowing the diver to breathe simultaneously from both.The manifold is a metal tube which is connected to the cylinder valves or directly to the cylinders, and may include an isolation valve or an outlet to connect a scuba regulator.