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A segmentary lineage society has equivalent parts ("segments") held together by shared values. A segmentary lineage society is a type of tribal society.
A close family is usually the smallest and closest segment and will generally stand together. That family is also a part of a larger segment of more distant cousins and their families, who will stand with each other when attacked by outsiders. They are then part of larger segments with the same characteristics. If there is a conflict between brothers, it will be settled by all the brothers, and cousins will not take sides. If the conflict is between cousins, brothers on one side will align against brothers on the other side. However, if the conflict is between a member of a tribe and a non-member, the entire tribe, including distant cousins, could mobilise against the outsider and his or her allies. That tiered mobilisation is traditionally expressed, for example, in the Bedouin saying: "Me and my brothers against my cousins, me and my cousins against the world."
The segmentary state has been used as a theoretical frame of reference for historical theories. For example, by Aidan Southall in "Illusion of Tribe" and by Burton Stein He has used the term to explain the polity of a number of empires.
Brian Schwimmer has described a system in which complementary opposition and genealogical principles of unilineal descent are used by residential groups as a basis for political mobilization in the absence of centralized political leadership.