The courts
stare decisis
The principle that courts follow their own earlier decisions. Latin for 'to stand by things decided'. It is a strong presumption, not an absolute rule.
In full
Lower courts must follow higher courts, and a court usually follows itself, so that law is predictable and people can rely on it. The Supreme Court can overrule itself, and the factors it cites include whether the old rule proved unworkable, whether facts or related law have changed, and how much reliance has built up around it. Every overruling is an argument about how much weight those factors carry.
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A plain-language explanation for civic education, not legal advice. When in doubt, the official source named above controls.