Liquid (or 'liquefied') petroleum gas (LPG) is a mixture of butane, propane and other light hydrocarbons produced either as a by-product of oil refining or from natural gas fields, and used as a vehicle fuel with lower CO2emissions than petrol. At standard temperature and pressure, it's a gas but the mixture can be cooled, or subjected to moderate pressure, and is then transformed into a liquid for ease of storage and transportation. LPG is used in dual-fuel vehicles, where it operates alongside a conventional petrol tank. It is also used in heating appliances.