A term applied to a sociocultural type or stage of human development. V. Gordon Childe proposed that the criteria for the urban revolution are: (1) cities, or large, dense settlements; (2) the differentiation of the population into specialized occupational groups; (3) social classes, including a ruling stratum exempt from primary subsistence tasks; (4) mechanisms for extracting a social surplus such as taxes or tribute; (5) monumental public buildings and other enterprises; and (6) writing.