relationship between a man and his sister's children, particularly her sons, that prevails in many societies. The term is derived from the Latin avunculus, meaning “uncle.” It typically involves for the maternal uncle a measure of authority over his nephews (and sometimes his nieces), coupled with specific responsibilities in their upbringing, initiation, and marriage. These children, in turn, often enjoy special rights to their uncle's property, often taking precedence in inheritance over the uncle's children.

Many societies that emphasize avunculate relationships also prefer cross-cousin marriages. In such societies, the ideal marriage unites the opposite-sex children of a sister and her brother—the sister's son and the brother's daughter—thus resolving property and inheritance issues by keeping them within the kin (kinship) group. Many avunculate cultures trace descent through the female line, a practice known as matrilineality, although some trace descent through the male line (patrilineality) or through both lines (bilateral descent).

In some societies, an arrangement known as avunculocal residence obtains, in which boys leave their natal homes during adolescence and join the household of one of their mother's brothers. Girls in these cultures generally remain in their mothers' homes until they marry, at which time they move to their husband's household. Hence, a long-established avunculocal joint family might include a married couple (or cowives and their husband), their unmarried daughters and preadolescent sons, and the husband's nephews and their wives, preadolescent sons, and daughters.