create social relationships; c) a gift binds both giver and recipient in a continuing social relationship Structures are universal; their realization is culturally specific. Lévi-Strauss argued that exchange is the universal basis of kinship systems, the structures of which depend on the type of marriage rules that apply. Because of its strong focus on vertical social relations, Lévi-Strauss' model of kinship systems came to be called alliance theory. Marriage Lévi-Strauss' model attempted to offer a single explanation for cross-cousin marriage, sister-exchange, dual organization and rules of exogamy. Over time, marriage rules create social structures because marriages are primarily forged between groups and not just between spouses. When groups exchange women on a regular basis, they marry together; consequently, each marriage creates a debtor/creditor relationship that must be balanced through the "repayment" of wives, either immediately or in the next generation. Lévi-Strauss proposed that the initial motivation for the exchange of women was the incest taboo. He deemed this the beginning and essence of culture, as it was the first prohibition to check natural impulses; secondarily, it divides labor by gender. Prescribing exogamy creates a distinction between marriageable and tabooed women that necessitates a search for women outside one's own kin group ("marry out or die out") and fosters exchange relationships with other groups. Exogamy promotes inter-group alliances and forms structures of social networks. Lévi-Strauss also discovered that a wide range of historically unrelated cultures had the rule that individuals should marry their cross-cousin, meaning children of siblings of the opposite sex—from a male perspective that is either the FZD (father's sister's daughter) or the MBD (mother's brother's daughter). Accordingly, he grouped all possible kinship systems into a scheme containing three basic kinship structures constructed out of two types of exchange. He called the three kinship structures elementary, semi-complex and complex. Elementary structures are based on positive marriage rules that specify whom a person must marry, while complex systems specify negative marriage rules (whom one must not marry), thus leaving room for choice based on preference. Elementary structures can operate based on two forms of exchange: restricted (or direct) exchange, a symmetric form of exchange between two groups (also called moieties) of wife-givers and wife-takers; in an initial restricted exchange FZ marries MB, with all children then being bilateral cross-cousins (the daughter is both MBD and FZD). Continued restricted exchange means that the two lineages marry together. Restricted exchange structures are