The Kyoto Protocol on Climate Protection extends not only to the greenhouse-gases CO2, methane and nitrous oxide, but also to the fluorinated gases (f-gases) HFC, PFC and SF6. By far the most important of the fluorinated greenhouse gases is HFC, used mainly as a substitute for CFCs, while the most significant of the HFCs themselves is HFC 134a, accounting for about three-quarters of world production. Since HFC 134a is mainly used as a refrigerant, it is also called R-134a, 'R' standing for 'refrigerant'.