This article studies in detail the settlement patterns of blacks in the urban North from before the Great Migration and through 1940, focusing on the cases of New York and Chicago. It relies on new and rarely used data sources, including census geocoded microdata from the 1880 census (allowing segregation patterns and processes to be studied at any geographic scale) and census data for 1900-1940 aggregated to enumeration districts. It is shown that blacks were unusually highly isolated in 1880 given their small share of the total population and that segregation reached high levels in both cities earlier than previously reported. Regarding sources of racial separation, neither higher class standing nor northern birth had much effect on whether blacks lived within or outside black neighborhoods in 1880 or 1940, and it is concluded that the processes that created large black ghettos were already in place several decades before 1940.