The courts
writ
A court's written command that something be done. Certiorari, habeas corpus, and mandamus are the three most consequential in public law.
In full
Writs are ancient instruments that survive because they let a court reach outside the ordinary appeal. A writ of certiorari calls a case up for review, habeas corpus tests the legality of detention, and mandamus orders an official to perform a clear duty. Extraordinary writs are granted rarely and only where no ordinary remedy will do.
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A plain-language explanation for civic education, not legal advice. When in doubt, the official source named above controls.