Gas cylinders are used to store gases under high pressures. Gas cylinders can store both flammable gases, such as acetylene, and inert gases such as helium. The three main types of compressed gases that are stored in gas cylinders are liquefied gas, non-liquefied gas, and dissolved gas. Gas cylinders can be defined by four main characteristics: type of gas, state of gas (compressed, refrigerated, dissolved, cryogenic liquid), purity of gas, and type of cylinder. Size and pressure differs from country to country.