In a 7-2 ruling in a case from Kentucky, the justices permitted states to exempt interest on their own bonds from taxation while taxing residents for interest on bonds issued by other states.

In the $2.5 trillion municipal bond market, 42 states exempt some or all interest on their bonds from income taxes, while taxing interest on bonds from other states.

The states have said that throwing out the system of exemptions that began 90 years ago would have a devastating impact on state finances.

Industry groups warned of possible turmoil in the municipal bond market if the existing setup were dismantled.

In the majority opinion, Justice David Souter said that the state tax exemptions go back to 1919 and have not hindered commerce among the states.

In dissent, Justice Samuel Alito said the majority decision "invites other protectionist laws."

Souter responded that that the dissent "rightly praises the virtues of the free market." But Souter said that overturning the tax exemptions now would upset the market in bonds based on the experience of nearly a century.