States, cities & tribes
Dillon's Rule
The default principle that local governments have only the powers the state expressly grants, plus those necessarily implied. Doubts are resolved against the locality.
In full
Named for an 1868 Iowa decision, Dillon's Rule treats local governments as creatures of the state with no inherent authority. Most states apply it to at least some localities, with home rule carving out exceptions. Whether a city may pass a local minimum wage or plastic bag ban often turns on which rule applies.
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A plain-language explanation for civic education, not legal advice. When in doubt, the official source named above controls.