In twelfth- and thirteenth-century Iceland, the vertical bond created through concubinage could prove to be a more long-lasting and reliable bond than the horizontal bond established through marriage. In the power struggles that characterized the period, concubinage – as well as friendship – proved to be important political instruments. However, the consolidation of the Church and the promulgation of its doctrine of monogamous and indissoluble marriage led to gradual changes in forms of cohabitation. Over the course of the thirteenth century, being married and simultaneously having one or more concubines became impossible, which in turn increased the political importance of well-organized marriages. In this chapter the correlation between the political development during the period of study and the importance of marriage in relation to other forms of cohabitation is scrutinized.