Also called Jim Crow, a moniker for slave, segregation was a system to enforce white–black, racial–ethnic separation and white supremacy.

Deep-rooted in customs and laws to control slaves and free blacks, it interfaced slavery, evolved after slavery's abolition, and was legal until 1964. In the North, where slave emancipation occurred ca. 1777–1846, blacks were separated by custom in churches, housing, jobs, schools, streetcars, and endured racist violence. Post–Civil War, only six northern states enacted black suffrage. In the South, domiciling more than 90 percent of African Americans, customary separation of the races morphed, legally and extralegally, into legalized apartheid. In face of Freedmen's Bureau efforts to educate and protect them, Black Codes and Ku Klux Klan terror restricted former slaves. Congress repealed the codes while approving states’ constitutions and readmissions that vitiated blacks’ citizenship, suffrage, and well-being. During Reconstruction and after, Democrats invented devices like the grandfather clause, poll tax, and registration reforms to suppress black and Republican voting; lynching; and convict leasing to abet separating blacks. Virginia declared a “white person” one who “has no trace whatever of any blood other than Caucasian.” A “colored person” was one “in whom there is ascertainable any Negro blood” (Lewis, 1999, p. 5). Schools, churches, businesses, industries, civic and social organizations, cohabiting and marriage, public auditoriums, hospitals, parks, prisons, and transportation thus were segregated years before the proliferation of Jim Crow laws.

Jim Crow codified rapidly during the 1890s. Democrats defeated Populist–Republican–Negro voter coalitions and constitutionally eliminated black voters in Mississippi (1890) and South Carolina (1895) as the Supreme Court's Plessy decision (1896) instituted the “equal, but separate” doctrine. Southern states hardly equalized black institutions and opportunities. They also winked at mob murder. From 1880 to 1930 alone 3,220 blacks and 723 whites were lynched in the South. The “white primary” election and poll tax continued. The Court overturned the former in 1944; the latter persisted to 1964–65.