29o AFRICAN POLITICAL SYSTEMS
There are probably never more than six sets in existence at any time, since six sets cover about seventy-five years. As each set dies its name is remembered only for a generation or two. Each set becomes more senior as the years go on, so that a man rises from a junior to a middle, and from a middle to a senior position in his community as a member of a group. The stratification of the age-set system is thus a further exemplification of the principle of segmentation which we have seen to be a characteristic of the political and kinship systems. There is further stratification within each set, but this is not of great importance, for the set sees itself, and is seen by others, as an undivided group in relation to other sets, and its divisions become merged as the set becomes more senior. A set once complete does not change its membership, but the sets are constantly changing their positions in relation to the whole system. There is also a certain relativity about these stratified sets similar to that we noted about tribal sections and clans, for, while they keep their distinction, there is often a situational fusion of two sets in relation to a third. This is especially apparent at feasts. Whether a set is regarded as junior or equal depends not only on its position in the age-set structure, but also on the status of a third set concerned in any situation, a tendency due to the connexion between age-sets and generations.
The most evident action of age-sets in determining behaviour is the way duties and privileges are effected by a transition from boyhood to manhood. Also, in virtue of the position of his set in the structure, every male Nuer is in a status of seniority, equality, or juniority towards every other Nuer man. Some men are his 'sons', some his 'brothers', and some his 'fathers'. Without entering here into further detail, we may say that the attitude of a man towards other men of his community is largely determined by their respective positions in the age-set system. Hence age relations, like kinship relations, are structural determinants of behaviour. The age-set system may, moreover, be regarded as a political institution, since it is, to a large extent, segmented tribally and since it divides a tribe—as far as its male members are concerned—into groups, based on age, which stand in a definite relation to each other. We do not consider, however, that it has any direct accord with the tribal structure, based on territorial segmentation, which we have recorded. The politico-territorial