Dillon Rule
If you're covering local government, you will hear a lot about the Dillon Rule.
Definition
The National League of Cities describes it this way:
Dillon's Rule is derived from written decision by Judge John F. Dillon of Iowa in 1868. It is a cornerstone of American municipal law. It maintains that a political subdivision of a state is connected to the state as a child is connected to a parent. Dillon's Rule is used in interpreting state law when there is a question of whether or not a local government has a certain power. Dillon's Rule narrowly defines the power of local governments.
The first part of Dillon's Rule states that local governments have only three types of powers:
those granted in express words,
those necessarily or fairly implied in or incident to the powers expressly granted, and
those essential to the declared objects and purposes of the corporation, not simply convenient, but indispensable.
The second part of Dillon's Rule states that if there is any reasonable doubt whether a power has been conferred on a local government, then the power has not been conferred. This is the rule of strict construction of local government powers.
Source
Clay Wirt. "Dillon's Rule." Virginia Town & City. August 1989, volume 24 number 8, pages 12 to 15.