In ancient Greece, a phratry (phratria, Greek: φ(ρ)ατρία, "brotherhood", "kinfolk", derived from φρατήρ meaning "brother") was a social division of the Greek tribe (phyle). Little is known about the role they played in Greek social life, but they existed from the Greek Dark Ages until the 2nd century BC.
The nature of the Greek phratries is, in the words of one historian, "the darkest problem among the [Greek] social institutions." 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 Greek 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 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).
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In anthropology, the Greek term is used to describe a unilineal descent group composed of a number of supposedly related clans which each retain their separate identities, but each feels some sort of special identity to the others within its phratry. A phratry may be formed when a clan grows so large that it splits up without losing their original (identity) connection.