The courts
rule of four
It takes four of the nine Justices to grant certiorari, not a majority. A minority can force the Court to hear a case the other five would rather avoid.
In full
The rule is an internal practice, not a statute. It exists so that a bare majority cannot control the docket entirely, and it means a grant of review is not a prediction of the outcome. Related conventions govern the fifth vote needed to stay an execution, since four votes to hear a case would otherwise be meaningless.
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A plain-language explanation for civic education, not legal advice. When in doubt, the official source named above controls.