Gas cylinders used for scuba diving come in various sizes and materials and are typically designated by material – usually aluminium or steel, and size. In the U.S. the size is designated by their nominal capacity, the volume of the gas they contain when expanded to normal atmospheric pressure. Common sizes include 80, 100, 120 cubic feet, etc., with the most common being the "Aluminum 80". In most of the rest of the world the size is given as the actual internal volume of the cylinder, sometimes referred to as water capacity, as that is how it is measured and marked (WC) on the cylinder (10 liter, 12 liter, etc.).