Kin Groups and Descent. Iatmul patrilineal clans ( ngaiva ) are the organizational basis of the social order. Most clans are organized into pairs, with one considered the elder brother and the other the younger, both tracing their origin to a pair of brothers who are the founding ancestors. Genealogies are important evidence of landownership, the right to produce and possess ritual paraphernalia and ritual knowledge, and the right to perform specific ceremonies. Clan membership also determines a man's place within the men's house. Within clans there exists a further differentiation into pairs of lineages, with the senior lineage having some authority over the junior one.
Kinship Terminology. Different terms are used for matrilateral and patrilateral kin. In each generation siblings of the same sex are classified together as are parallel cousins, and in the parent's generation affinal relatives are addressed in terms used also for consanguineal kin.