DIN 51622 (December 1985) gives this definition: "Liquefied petroleum gases are understood to be the standard technical grades of C3 and C4 hydrocarbons – propane, propene, butane, butene – and their mixtures." It is a relatively easy matter to liquefy these gases, which arise during the production and processing of mineral oil and natural gas, during the synthesis of hydrocarbons, and in petrochemical processes. The term liquefied petroleum gases is often used also for such gases which can be liquefied only by means of cryogenic technology and/or under high pressures. See cryogenic seals for information about sealing liquefied petroleum gases and liquefied gases.