The heart of the main power conversion system of the steam engine. A locomotive has at least two cylinders, mounted at the leading end so as to be clear of the driving wheels. Next to each cylinder is a valve which controls the flow of steam into and out of it. Normally, a cylinder and its valve chest are cast in a single block which is carried on one of the side frames. A locomotive may have two, three or four cylinders depending on the design and age, but there will always be at least two. On some two-cylinder locomotives, the cylinders are hung between the frames ("inside cylinder locomotives") and drive the wheels through a cranked axle. With this design the cylinders and valve gear are largely hidden from the outside and are difficult to see unless you know where to look. Locomotives with three or four cylinders will have two outside and the others inside the frames. Much importance is laid on the size of cylinders in relation to the boiler size and pressure and the amount of work the locomotive is required to do. See also Cylinder Operation.