A group or tribe of persons from one common family, united by a chieftain. A clan develops for social and security reasons and membership of a clan is defined in terms of actual or purported descent from a common ancestor. The descent is unilineal - i.e., derived only through the male (patriclan) or the female (matriclan). Normally, but not always, the clans are exogamous, marriage within the clan being forbidden and regarded as incest. Clans may segment into subclans or lineages.