The City of Valdez in Alaska imposed a property tax that, in practice, applied only to large oil tankers.

Polar Tankers, a ConocoPhillips subsidiary, sued, arguing that the tax violated the Tonnage Clause of the Constitution, which forbids a state, "without the Consent of Congress, [to] lay any Duty of Tonnage."

The court noted that the original meaning of the clause was as a term of art for a tax imposed that varies with "the internal cubic capacity of a vessel," but emphasized that a consistent line of cases had read the clause broadly, "forbidding a State to do that indirectly which she is forbidden ... to do directly.

"[1] Those cases, the court concluded, stood for the proposition that the tonnage clause's prohibition reaches any taxes or duties on a ship, "whether a fixed sum upon its whole tonnage, or a sum to be ascertained by comparing the amount of tonnage with the rate of duty[,] ... regardless of [the tax or duty's] name or form ... which operate to impose a charge for the privilege of entering, trading in, or lying in a port.

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