The rapid influx of blacks during the Great Migration disturbed the racial balance within Northern and Western cities, exacerbating hostility between both blacks and whites in the two regions. Deed restrictions and restrictive covenants became an important instrument for enforcing racial segregation in most towns and cities, becoming widespread in the 1920s. Such covenants were employed by many real estate developers to "protect" entire subdivisions, with the primary intent to keep "white" neighborhoods "white". Ninety percent of the housing projects built in the years following World War II were racially restricted by such covenants. Cities known for their widespread use of racial covenants include Chicago, Baltimore, Detroit, Milwaukee, Los Angeles, Seattle, and St. Louis.