A soil classification scheme that was developed between 1961 and 1974 by the Food and Agriculture Organization of the UN (FAO) and the UN Educational, Scientific, and Cultural Organization (UNESCO), supported by the UN Environment Programme (UNEP) and the International Society of Soil Science. It has been revised several times. In 1998 the system was named the World Reference Base for Soil Resources. The WRB divides soils into 32 soil groups and 170 subunits. The soil groups are: acrisols, albeluvisols, alisols, andosols, anthrosols, arenosols, calcisols, cambisols, chernozems, cryosols, durisols, ferralsols, fluvisols, gleysols, gypsisols, histosols, kastanozems, leptosols, lixisols, luvisols, nitisols, phaeozems, planosols, plinthosols, podzols, regosols, solonchaks, solonetz, stagnosols, technosols, umbrisols, and vertisols.