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:Second, the Court in Scheiner also held<ref>483 U.S. at 284-285.</ref> that the challenged flat taxes violated the "internal consistency" prong of the "fair apportionment" requirement that the Court has applied to state taxes levied on interstate commercial activity.  The Scheiner Court explained that, "if each State imposed flat taxes for the privilege of making commercial entrances into its territory, there is no conceivable doubt that commerce among the States would be deterred."<ref>483 U.S. at 284.</ref> Scheiner's discussion of internal consistency was closely linked to its determination that the Pennsylvania flat taxes discriminated against out-of-state carriers.<ref>''See id.'' at 285-286.</ref>

:Second, the Court in Scheiner also held<ref>483 U.S. at 284-285.</ref> that the challenged flat taxes violated the "internal consistency" prong of the "fair apportionment" requirement that the Court has applied to state taxes levied on interstate commercial activity.  The Scheiner Court explained that, "if each State imposed flat taxes for the privilege of making commercial entrances into its territory, there is no conceivable doubt that commerce among the States would be deterred."<ref>483 U.S. at 284.</ref> Scheiner's discussion of internal consistency was closely linked to its determination that the Pennsylvania flat taxes discriminated against out-of-state carriers.<ref>''See id.'' at 285-286.</ref>

The Court's reference to the possible cumulative impacts of multiple flat taxes, however, suggests the related but distinct concern that a proliferation of such taxes would impose increasing burdens on vehicles operating in a large number of States, thus impeding the ability of interstate carriers to engage in unfettered "travel within the free trade area" established by the Commerce Clause.<ref>''Id.'' at 284; ''see id.'' at 285 nn.19 & 20.</ref>

The Court's reference to the possible cumulative impacts of multiple flat taxes, however, suggests the related but distinct concern that a [[proliferation]] of such taxes would impose increasing burdens on vehicles operating in a large number of States, thus impeding the ability of interstate carriers to engage in unfettered "travel within the free trade area" established by the Commerce Clause.<ref>''Id.'' at 284; ''see id.'' at 285 nn.19 & 20.</ref>

Justice [[John Paul Stevens]] delivered the opinion of the court, joined by Justices [[William Brennan]], [[Byron White]], [[Thurgood Marshall]], and [[Harry Blackmun]].  Justices [[Sandra Day O'Connor]] (joined by Justices [[William Rehnquist]] and [[Lewis Powell]]) and [[Antonin Scalia]] (joined by Justice Rehnquist) filed dissenting opinions.

Justice [[John Paul Stevens]] delivered the opinion of the court, joined by Justices [[William Brennan]], [[Byron White]], [[Thurgood Marshall]], and [[Harry Blackmun]].  Justices [[Sandra Day O'Connor]] (joined by Justices [[William Rehnquist]] and [[Lewis Powell]]) and [[Antonin Scalia]] (joined by Justice Rehnquist) filed dissenting opinions.