Descent and alliance theories are two foundational, yet distinct, approaches in anthropological kinship studies, offering different perspectives on how societies are structured through marriage and kinship.

Descent Theory: Descent theory, primarily associated with British structural-functionalists like A.R. Radcliffe-Brown, E.E. Evans-Pritchard, and Meyer Fortes, emphasizes the role of unilineal descent groups (patrilineal or matrilineal) as the primary building blocks of social structure. Its cardinal points include:

1. Unilineal Descent: Focuses on societies where kinship is traced exclusively through either the male line (patrilineal) or the female line (matrilineal). This creates clearly defined, corporate groups.
2. Corporate Groups: Descent groups are seen as enduring, corporate entities that hold rights and responsibilities collectively, such as land ownership, political office, and ritual duties. They provide social identity, regulate marriage, and maintain social order.
3. Social Structure: Kinship is viewed as the framework for political and economic organization. Descent groups are the stable, enduring units that provide continuity and structure to society, often transcending individual lives.
4. Exclusionary Principle: Membership in a descent group is often exclusive, determining who belongs and who does not, thereby defining rights and obligations within the group.

Alliance Theory: Alliance theory, championed by Claude Lévi-Strauss, shifts the focus from descent to marriage as the fundamental mechanism for creating social structure. It emphasizes the exchange of women between groups, which creates alliances and fosters social cohesion. Its cardinal points include:

1. Marriage as Exchange: Marriage is seen not merely as a union between individuals but as a reciprocal exchange between groups (families, lineages, clans). This exchange creates enduring alliances and interdependence.
2. Incest Taboo and Exogamy: The universal incest taboo compels groups to seek marriage partners outside their immediate kin (exogamy). This necessity drives the formation of alliances between otherwise distinct groups.
3. Reciprocity: Alliance theory highlights the principle of reciprocity in marriage exchanges. This can be direct (e.g., sister exchange) or generalized (e.g., a group gives women to another and receives women from a third, forming a chain).
4. Elementary vs. Complex Structures: Lévi-Strauss distinguished between 'elementary structures' (where marriage rules explicitly define who one must marry, e.g., cross-cousin marriage) and 'complex structures' (where rules only specify who one cannot marry, leaving a wider choice).
5. Kinship as Communication: Kinship systems are viewed as systems of communication, where women are the 'messages' exchanged between groups, creating a network of social relations.

In summary, descent theory emphasizes the internal solidarity and corporate nature of groups formed by shared ancestry, while alliance theory focuses on the external relations and interdependence created through marriage exchanges between groups. Both have been crucial for understanding the diverse forms of kinship organization across human societies.