Schools have functioned as primary sites for civil rights protest and action throughout the United States. Prior to the 1954 Brown v. Board of Education Supreme Court decision declaring legalized segregation in public schools unconstitutional, laws existed in the South and border states requiring black and white students to attend separate schools. No such laws existed during this period in the North, yet many school systems in the urban North were segregated nonetheless. The existence of segregated public schools in the North was caused primarily by "neighborhood school" policies which strictly assigned students to schools within their own neighborhoods. Because so many neighborhoods in the North were racially segregated, neighborhood school policies resulted in a corresponding racial segregation of the schools. Additionally, school boards adopted other measures which served to strengthen the segregation in the school system rather than weaken it. In Milwaukee, these measures included the practice of intact busing and the selection of school district boundaries in locations which would result in racially segregated districts. Racial segregation caused by discriminatory practices, instead of by law, is called de facto segregation.