In many societies marriage links not just nuclear families but larger social formations as well. Some endogamous societies are divided into different exogamous groups (such as clans or lineages): Men form alliances through the exchange of women, and the social organization regulates these alliances through marriage rules. In some cases, two men from different groups exchange sisters for brides. Other instances involve an adult man marrying the young or infant daughter of another man; sexual relations would be deferred for many years, but the two men will have formed a strong bond. Marriages are often arranged by the families through the services of a matchmaker or go-between, and commence with a ritual celebration, or wedding. Some cultures practice trial marriage; the couple lives together before deciding whether they should marry. Society generally prescribes where newlywed couples should live: In patrilocal cultures, they live with or near the husband's family; in matrilocal ones, with or near the wife's family. Under neolocal residence, the couple establishes their own household.
Although marriage tends to be regarded in many places as a permanent tie, divorce is allowed in most modern societies. The causes of divorce vary, but adultery, desertion, infertility, failure to provide the necessities of life, mistreatment, and incompatibility are the most common. Civil unions are now permitted in Western countries, but for nearly a thousand years marriage in the Western world was a religious contract. The Christian church undertook its supervision in the 9th cent., when newlywed couples instituted the practice of coming to the church door to have their union blessed by the priest. Eventually the church regulated marriage through canon law.
In contemporary Europe marriage has lost some of importance, especially as social legislation in some nations has emphasized assuring equal financial benefits and legal standing to children born to unwed parents. Some European nations also grant legal recognition to less restrictive unions between a man and a woman; such partnerships typically have some but not all of the legal rights extended to married couples, but the partnership usually can be more easily dissolved.