The Tiv of Northern Nigeria, who number some 800,000, believe that they are all descended from one man, Tiv, through some 14 to 17 generations of known ancestors (Fig. 1). These ancestors are constantly named in casual conversation and in discussion of serious affairs. To understand things Tiv one must know Tiv genealogies. By genealogical reference a Tiv traces ties of kinship and marriage, claims a place to live and farm, argues his case in a moot, conducts matters of magic and ritual, and decides against whom he will fight on any given occasion. Genealogies are the key to Tiv social organization. Tiv political structure is based on a lineage system: a system of segmented groups in which each segment (save the maximal) is included in a segment of a higher order but of the same kind, and each segment (save the minimal) includes segments of a lower order but of the same kind. The Tiv system has an almost one to one correlation between lineage and territorial segments. Both are called ‘those of X’, the eponymous ancestor; thus Mbaduku (those of Aduku) describes both the lineage segment (ipaven, pl. uipaven) and the discrete territory which they inhabit (tar, pl. utar).