What did the Supreme Court's decision in the Plessy v. Ferguson court case illustrate about the relationship between black and white Southerners as well as racial problems outside the American South?

The Plessy v. Ferguson (1896) decision reinforced the racial status quo in the South while largely impacting and legitimizing the practice of racial segregation.
Under Louisiana law, train cars were segregated by race. Homer Plessy, a black man in New Orleans, Louisiana, sat in a “whites only” train-car and refused to move to the “blacks only” train-car. (Plessy had one-eighth black ancestry.) Plessy was removed and arrested. Plessy appealed his conviction, and the case rose to the United States Supreme Court. The Supreme Court sided against Plessy, upholding the racial segregation laws in Louisiana and subsequently all racial segregations laws in the South. The Supreme Court upheld the “separate but equal” doctrine, which argued that states could segregate the races into separate facilities, as long as those accommodations were equal. Unfortunately, “black” facilities were hardly (if ever) equal to “white” facilities.
The Plessy decision reinforced racial segregation across the American South. Many southern states began racially segregating facilities in the 1870s and 1880s, but the Plessy decision legitimized the segregation many southern states had put in place. As a result, many southern states segregated all public places, including buses, trains, hospitals, hotels, schools, movie theaters, and restaurants.
While racial segregation is most commonly associated with the South, many other states used the Plessy decision to segregate public places. The Plessy case allowed all areas of the country to segregate based on race. For example, the Supreme Court case that ended segregation in public schools (Brown v. Board of Education) originated in Kansas.