Once the boiler has generated sufficient steam, it can be used for useful work. A valve fitted on top of the boiler and often housed in a dome, is used by the driver to admit steam to the cylinders. The valve is called "the regulator" (known as the "throttle" in the US) and is opened and closed by means of a long shaft connected to a lever accessed from the driver's position in the locomotive cab. The steam collected in the dome can be admitted, by use of the regulator, into a steam pipe which is connected to the cylinders. Some locomotives have superheated steam and in such cases the regulator may be located in the smokebox. The regulator is controlled from the cab by a lever. UK practice is to mount the lever on the top centre of the firebox backplate so that it is moved clockwise or anticlockwise to open or close the regulator valve. The shaft connecting the lever to the regulator valve passes through the boiler steam space. An alternative form of regulator control has the operating rods mounted on the outside of the boiler (along the left or right side) and actuated by a forward and aft lever in the cab. This type was popular in the US but was used on many of the more modern locomotives in the UK. It should be noted that just because the operating rods are visible on one side of the boiler, it will not necessarily follow that the driving position is on that side. Some locomotives have the driving position on the other side with the regulator handle connected to the operating rods by a cross shaft. In some cases, regulator handles were provided on both sides of the cab.