Cultures transform cross-culturally equivalent genealogies into socially defined kin relations by the way in which they name categories of individuals who are members of the kin group. Such naming results in the kinship terminology of the culture. There are two basic sets of kin terms: terms of reference and terms of address. A term of reference is how ego would refer to that relative in communications with others. For example, if ego were asked "Who is that person to you?" he or she might say "That is my father." A term of address is what one calls that person when interacting with him or her—e.g., "Hi, Dad." In all cultures, one or more of nine basic criteria are used in the system of kinship terminology particular to that culture (Kroeber 1909; Murdock 1949): generation, lineality and collaterality, sex, affinity, polarity, bifurcation, relative age, speaker's sex, and address versus reference. In the United States and most other Western societies, the first five criteria are commonly used. North Americans customarily distinguish among kin and assign kin terms on the basis of a person's generation, directness of relationship, sex, ties of blood versus marriage, and the use of different terms by interacting kin.
Different cultures collapse different relatives under one name that allows kinship terminology to transform objectively identical genealogies into different social constructions of kinship. For instance, the kinship terminology employed in the United States uses the term aunt to refer to all of the sisters of a person's mother but employs a different term for the mother herself. In some other cultures, by contrast, a person's mother and the mother's sister are referred to by the same term. Relatives who are called by the same label tend to be identified with similar roles, responsibilities, and privileges with regard to ego. Similarly, relatives who are distinguished from each other terminologically also tend to play distinctive roles with respect to ego. Kin names, therefore, act to reinforce cultural expectations about how kin will behave toward one another. While classificatory kin terms emphasize similarities in the relationships of different kin to ego, individuals distinguish between relatives who are called the same name and respond to them differently in a myriad of ways. Thus, in societies where mother and mother's sisters are called by the same term, children know the difference between their mothers and aunts, treat them differently, and feel differently about them. The collapsing of different categories of relatives under one label facilitates certain kinds of interactions between kin but does not eradicate an individual's ability to appreciate that people called by the same name are, nevertheless, not the same people.
While kin terminology is not uniform across cultures, there are a number of systems of kin naming that appear over and over from one culture to the next. Six such systems of kinship terminology have been identified, based on the manner in which cousins and siblings are classified: Hawaiian, Eskimo, Sudanese, Iroquois, Crow, and Omaha (Murdock 1949). U.S. kin terms are of the Eskimo type. Cousins are distinguished from brothers and sisters, but cousins on the father's side are not distinguished from those on the mother's side—they are all referred to as cousin.
Cultures that share systems of kinship terminology also tend to be similar in residence patterns, descent rules, and family organization (Levinson and Malone 1980). These similarities in important features of social structure are thought to account for shared kin terminology systems. Societies with similar patterns of descent, residence, and family organization are likely to allocate roles, rights, and responsibilities similarly.
A shared system of kin terminology reflects and reinforces these similar role assignments. For example, in societies that trace descent through the father, a greater number of terminological distinctions are made regarding relatives from different generations for kin traced through the father than for kin traced through the mother. This may be because role, rights, and responsibilities depend upon the age of the relative vis-à-vis ego. As the interactions between an individual and the father's kin are more finely enumerated and distinguished in cultures where descent is traced through the male parent, the generation-based name distinctions on the father's side of the genealogy that are typical of these cultures reflect generation-based role distinctions. As interactions between an individual and relatives traced through the mother are not so finely drawn, terminological distinctions also tend to be less finely distinguished.
In societies that trace descent through the father, married couples also tend to live with or near the husband's family. This means that children of both sexes as well as married males will be interacting daily with relatives traced through the father, while no individuals will ever live where there is a concentration of relatives traced through the mother. The finer distinctions between the father's kin on the basis of age may reflect the far greater number of interactions that an individual will have with these relatives and, therefore, the greater need to distinguish these relatives on the basis of age. Societies that reverse this pattern of kin naming, distinguishing between mother's but not father's kin on the basis of generation, tend to trace descent through the female parent. In cultures of this kind, married couples are more likely to live with or near the wife's family. The greater role of the mother's kin in the life of the individual is mirrored in the more clearly differentiated kin labeling with respect to relatives traced through the mother.