Cite as: 514 U. S. 175 (1995)
Opinion of the Court
one State exceeded its fair share would be taxed again by a State properly laying claim to it.
For over a decade now, we have assessed any threat of malapportionment by asking whether the tax is "internally consistent" and, if so, whether it is "externally consistent" as well. See Goldberg, supra, at 261; Container Corp., supra, at 169. Internal consistency is preserved when the imposition of a tax identical to the one in question by every other State would add no burden to interstate commerce that intrastate commerce would not also bear. This test asks nothing about the degree of economic reality reflected by the tax, but simply looks to the structure of the tax at issue to see whether its identical application by every State in the Union would place interstate commerce at a disadvantage as compared with commerce intrastate. A failure of internal consistency shows as a matter of law that a State is attempting to take more than its fair share of taxes from the interstate transaction, since allowing such a tax in one State would place interstate commerce at the mercy of those remaining States that might impose an identical tax. See Gwin, White & Prince, 305 U. S., at 439. There is no failure of it in this case, however. If every State were to impose a tax identical to Oklahoma's, that is, a tax on ticket sales within the State for travel originating there, no sale would be subject to more than one State's tax.
External consistency, on the other hand, looks not to the logical consequences of cloning, but to the economic justification for the State's claim upon the value taxed, to discover whether a State's tax reaches beyond that portion of value that is fairly attributable to economic activity within the taxing State. See Goldberg, supra, at 262; Container Corp., supra, at 169-170. Here, the threat of real multiple taxation (though not by literally identical statutes) may indicate a State's impermissible overreaching. It is to this less tidy world of real taxation that we turn now, and at length.
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