Agnatic is an anthropological term which refers to systems that trace kinship and genealogical descent through males alone (as opposed to cognatic, which trace such ties through both male and female members of a lineage). Nuer society is typically organized around a system of agnatic lineages, which together make up the clan structure of Nuer kinship.

A clan is the largest unit of Nuer social structure that can be represented as a union of lineages tied together by a common agnatic kinship structure. While clans are less likely than lineages to be categories used in a Nuer’s expression of self-identification, their boundaries can be deduced by the rules of exogamous marriage which apply to them (i.e., those rules which specify the group beyond whose boundaries one must find a mate). Clans are not to be confused with tribes, as clans are a kinship system of identification, and may be dispersed throughout geographical areas in various ways, while tribes are a political system of identification and often include an element of territorial interest. Further, men usually seek wives from within their tribe, but are barred by rules of exogamy from seeking a wife from within their clan. Clans are composed of agnatic lineages which branch down from maximal lineages to minimal lineages, at which point individual family-units come into view.