The question “Who marries whom?” refers to patterns of partner choice. The tendency to marry (or to enter a long-term relationship such as cohabitation with) a person who belongs to the same social group, or who is similar with regard to certain characteristics, is also known as homogamy. Sociologists have traditionally been interested in three individual characteristics that can be important in the choice of a partner: race/ethnicity, religion, and socioeconomic resources. Studying patterns of partner choice is important because it allows us to evaluate the degree of openness of the boundaries of different ethnic, religious, and socioeconomic groups. The more frequent marriage between subjects who differ with respect to the characteristics of the group is, the more open the group.