In pre-Classical Greece, each tribe (phyle) was divided into phratries. The nature of these phratries is, in the words of one historian, "the darkest problem among the [Greek] social institutions. Little is known about the role they played in Greek social life, but they existed from the Dark Ages until the second century BC; Homer refers to them several times, in passages that appear to describe the social environment of his times. In Athens, enrollment in a phratry seems to have been the basic requirement for citizenship in the state before the reforms of Cleisthenes in 508 BC. From their peak of prominence in the Dark Ages, when they appear to have been a substantial force in Greek social life, phratries gradually declined in significance throughout the classical period as other groups (such as political parties) gained influence at their cost.
Phratries also exist among Native Americans, among whom a phratry was often identified by a nature sign. In some cultures, such as the Tlingit, and the Lenape, intermarriage between phratries was mandated. Phratries contained smaller kin groups called gene; these appear to have arisen later than phratries, and it appears that not all members of phratries belonged to a genos; membership in these smaller groups may have been limited to elites. On an even smaller level, the basic kinship group of ancient Greek societies was the oikos (household).