In "Municipal Corporations" (1872), Dillon explained that in contrast to the powers of states, which are unlimited but for express restrictions under the state or federal constitution, municipalities only have the powers that the state legislature expressly gives to them. This formulation of the scope of municipal power came to be known as the "Dillon Rule". It says that municipal governments only have the powers that are expressly granted to them by the state legislature, those that are necessarily implied from that grant of power, and those that are essential and indispensable to the municipality's existence and functioning. If the legislative grant of power is unclear, it should be read narrowly. Courts should assume that the legislature did not mean to give a power unless that power was clearly spelled out in a law. However, when the state has not specifically directed the method by which the municipality may implement its granted power, the municipality has the discretion to choose the method so long as its choice is reasonable.