Kinship and descent is one of the major concepts of cultural anthropology. Cultures worldwide possess a wide range of systems of tracking kinship and descent. Anthropologists break these down into simple concepts which are common among many different cultures.
A descent group is a social group whose members claim common ancestry. A unilineal society (such as the Iroquois system) is one in which the descent of an individual is reckoned either from the mother's or the father's descent group. With matrilineal descent individuals belong to their mother's descent group. With patrilineal descent, individuals belong to their father's descent group.
In a society which reckons descent bilineally (such as the Eskimo system), descent from both father and mother is equally important.
Societies can also consider descent to be ambilineal (such as Hawaiian system) where offspring determine their lineage through the matrilineal line or the patrilineal line.
Lineages, clans, phratries and moieties
A lineage is a descent group who can demonstrate their common descent from an apical ancestor. Lineages can be matrilineal or patrilineal, depending on whether they are traced through mothers or fathers, respectively. Whether matrilineal or patrilineal descent is considered most significant differs from culture to culture.
A clan is a descent group that claims common descent from an apical ancestor but cannot demonstrate it (stipulated descent). If a clan's apical ancestor is nonhuman, it is called a totem. Examples of clans are Scottish, Irish, Chinese and Japanese clans.
A phratry is a descent group containing at least two clans which have a supposed common ancestor.
If a society is divided into exactly two descent groups, each is called a moiety, after the French word for half.
The nuclear family
The Western model of a nuclear family consists of a couple and their children. The nuclear family is ego-centered and impermanent, while descent groups are permanent (lasting beyond the lifespans of individual constituents) and reckoned according to a single ancestor.
Kinship calculation is any systemic method for reckoning kin relations. Kinship terminologies are native taxonomies, not developed by anthropologists.
The notion of the nuclear family has been largely dismissed, as have other theories which argue for a universal core unit of kinship.
Kinship and descent have a number of legal ramifications, which vary widely between legal and social structures.
Most human groups share a taboo against incest; which relatives are forbidden from marriage by the rules tend to vary widely once you move beyond the nuclear family. At common law, the prohibitions are typically phrased in terms of "degrees of consanguinity."
More importantly, kinship and descent enters the legal system by virtue of intestacy, the laws that at common law determine who inherits the estates of the dead in the absence of a will. In civil law countries, the doctrine of legitime plays a similar role, and makes the lineal descendants of the dead person forced heirs. Rules of kinship and descent have important public aspects, especially under monarchies, where they determine the order of succession, the heir apparent and the heir presumptive.