<p>While racial segregation is prevalent both within U.S. cities and across metropolitan areas, relatively little has been done to examine ethnic differences along the municipal borders where city and suburb meet. Because of economic factors such as sorting, or political ones such as redlining, school segregation, or restrictive covenants, neighborhood racial makeup might change sharply at a city border. In this study, Census data are used to calculate block-group-level diversity for a set of two dozen large U.S. cities and their contiguous suburbs, before developing a mathematical method to calculate a “Border Disparity Index” that varies by MSA, core city, and suburb. This index can be compared across the sample to examine which core cities are more likely to have borders that separate more-diverse block groups from less-diverse ones, which core cities are relatively more or less diverse than their suburbs on average, and which individual suburbs have the largest disparities vis-à-vis their core cities. In particular, Atlanta and Detroit have particularly diverse suburbs, while Milwaukee and Portland do not. Income differences and suburban shares of Black residents play significant roles in driving differences between suburbs.</p>