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Rh Sebelius, 997 F. Supp. 2d 415 (ED Va. 2014). The Court of Appeals for the Fourth Circuit affirmed. 759 F. 3d 358 (2014). The Fourth Circuit viewed the Act as "ambiguous and subject to at least two different interpretations." Id., at 372. The court therefore deferred to the IRS’s interpretation under Chevron U. S. A. Inc. v. Natural Resources Defense Council, Inc., 467 U. S. 837 (1984). 759 F. 3d, at 376.

The same day that the Fourth Circuit issued its decision, the Court of Appeals for the District of Columbia Circuit vacated the IRS Rule in a different case, holding that the Act "unambiguously restricts" the tax credits to State Exchanges. Halbig v. Burwell, 758 F. 3d 390, 394 (2014). We granted certiorari in the present case. 574 U. S. ___ (2014).

The Affordable Care Act addresses tax credits in what is now Section 36B of the Internal Revenue Code. That section provides: “In the case of an applicable taxpayer, there shall be allowed as a credit against the tax imposed by this subtitle. . . an amount equal to the premium assistance credit amount.” 26 U. S. C. §36B(a). Section 36B then defines the term “premium assistance credit amount" as "the sum of the premium assistance amounts determined under paragraph (2) with respect to all coverage months of the taxpayer occurring during the taxable year." §36B(b)(1) (emphasis added). Section 36B goes on to define the two italicized terms—"premium assistance amount" and "coverage month"—in part by referring to an insurance plan that is enrolled in through "an Exchange established by the State under [42 U. S. C. §18031]." 26 U. S. C. §§36B(b)(2)(A), (c)(2)(A)(i).

The parties dispute whether Section 36B authorizes tax credits for individuals who enroll in an insurance plan through a Federal Exchange. Petitioners argue that a