A gas cylinder or tank is a pressure vessel at above atmospheric pressure for storage and containment of gases. Also called bottles are high-pressure gas cylinders. Depending on the physical characteristics of the contents, the stored contents may be in a state of compressed gas, vapor over liquid, supercritical fluid, or dissolved in a substrate material. A typical design of the gas cylinder is elongated, standing upright on a flattened bottom end, with the valve at the top to connect to the receiving device.