As if the IRS weren’t reeling enough already, today the unanimous Supreme Court dealt the beleaguered agency another blow, unanimously ruling that companies who paid a British “windfall tax” could get credit for that payment against their U.S. tax liabilities. This should’ve been a simple case, and the federal tax court got it right — the tax code credits foreign income taxes — but the court of appeals found a convoluted way to rule for the IRS.

As Cato’s brief explained, however, taxpayers have the right to be free from double taxation and here the IRS improperly disregarded the substance of the windfall tax. A foreign tax’s form or label can’t mask its substantive character for legal purposes. American businesses operating overseas should be able to rely on a stable, substantive application of U.S. tax law instead of arbitrary interpretations and constructions manipulated to generate payments to the IRS.

The Supreme Court had to invoke and explain complicated equations to reach its decision — I’ve never seen so much math in an opinion — but this ruling ultimately boils down to the longstanding doctrine regarding how to evaluate a tax: (1) A tax’s “predominant character,” or the normal manner in which it applies, controls what kind of tax it is for other legal purposes; and (2) foreign tax creditability depends not on the way a foreign government characterizes its tax but on is economic effect — whether the tax, if enacted in the United States, would be an income tax or something else.

That’s the big takeaway here: The specific since-repealed UK tax at issue in PPL Corp. v. Commissioner of Internal Revenue isn’t likely to come up again, but the IRS is on notice that it doesn’t have discretion to err in favor of the Treasury whenever it feels like it. The tax code provides rules –albeit often overly complicated ones — that courts will enforce.