Ruling in the landmark decision Plessy v. Ferguson, the U.S. Supreme Court upheld the legality of racial segregation in public transportation under the "separate but equal" doctrine.

The following landmark court decisions changed the interpretation of existing law in the United States. Such a decision may settle the law in more than one way:

• establishing a new legal principle or concept;
• overturning precedent based on its harmful effects or flaws in its reasoning;
• distinguishing a new principle that refines an existing principle, thus departing from prior practice without violating the rule of stare decisis;
• establishing a test or a measurable standard that can be applied by courts in future decisions.