The phrase "separate but equal" was actually drawn from the Louisiana law itself that was the subject of the Plessy v. Ferguson case. The 1890 law said that railroads were required to provide “equal but separate accommodations for the white, and colored, races.” On June 7, 1892, Homer Plessy refused to move from a seat reserved for whites (he was 1/8 black). The judge overseeing the case, Judge John H. Ferguson, upheld the law. The case moved its way up to the Supreme Court. The infamous decision of Plessy v. Ferguson was announced in 1896, and the the U.S. Supreme Court ruled that segregation in America was constitutional, or that the "equal protection" clause of the United States constitution was satisfied if "separate but equal" accommodations/services were provided for both whites and blacks.

When courts refer to a legal doctrine, they are referring to the principle that is established by the case itself. "Separate but equal" became a legal doctrine, or a principle, that justified much more than railroad segregation laws. It was used to justify nearly all of the hallmarks of segregation: separate universities for blacks and whites; separate schools for young white and black children; separate seating areas for transportation; separate drinking fountains; the denial of services at public facilities operated by white people (restaurants, hotels, etc...) to African Americans. This doctrine continued to justify segregation until another landmark US Supreme Court decision, Brown v. Board of Education, overruled Plessy v. Ferguson and ordered that schools were to be desegregated.