Marriage, or the lack thereof, between members of different social groups has long been viewed as an indicator of the permeability of group boundaries and the rigidity of the stratification system. Drawing on statistical and computational methods as well as original in-depth interviews, I examine marriage sorting patterns across various forms of social and symbolic boundaries, in order to understand marriage’s implications for social openness and closure across societies. Specifically, I focus on racial/ethnic intermarriage in the United States, and rural-urban marriage in China.