The United States Constitution is said to be a "living document" in part because court interpretations change over time. What was a key aspect of Plessy v. Ferguson, a Supreme Court decision that was later overturned?

The key aspect of that sentence was to affirm the notion that racial segregation was not unconstitutional. The rule of Plessy v. Ferguson states that separated facilities for different racial groups (actually for whites and for blacks) don't violate the US Constituion as far as they are equal, that is, they offer equal conditions. The initial complaint had to do with a public pool. The sentence was overturned by the 1954 Brown v. Board of Education regarding separate schools for black for white children; it held that even under equal conditions, segregated schools were unconstitutional.