There aren't many occasions when citizens are forced to reckon with the liminal strangeness of license plates. They're legally required to operate a car, a burden placed on any citizen who wishes to drive, yet also, increasingly, a space for highly personal self-expression. It's the spot where one can declare allegiance to an alma mater, affection for wildlife, or disdain for cancer. Other drivers use them to list names, make jokes, or with bizarre frequency, to state the make and model of their car.

That dual purpose is confusing to think about, but it's also legally hazardous. Let's say you grow up in Ohio, rooting for the Ohio State University. Then you get a job over the border in Ann Arbor, home of the University of Michigan. When you register your car, you decide you want a vanity license plate: GOBUCKS, to declare your football allegiance.

Whose sentiment, offensive to many Michiganders, does that represent? Yours, or the state of Michigan's? Or, to put that another way, if there's a question of free speech, is it your speech or the government's? The Supreme Court heard arguments Monday in Walker v. Sons of Confederate Veterans, a case that centers on this question.

The material in question is even more explosive than the Ohio State-Michigan rivalry. The case concerns the Sons of Confederate Veterans, a group that either celebrates Southern heritage or is a neo-Confederate hate group, depending on who you ask. SCV wanted Texas to offer vanity plates with the SCV logo, which includes the flag of the Army of Northern Virginia—the symbol commonly associated with the Confederacy. Texas rejected the license plate, saying: “A significant portion of the public associates the Confederate flag with organizations advocating expressions of hate directed toward people or groups that is demeaning to those people or groups.” (Nine states offer SCV plates, though Maryland and Virginia do so only under court orders stemming from previous litigation.)

The legal challenges to that decision have split the typical left-right politics in the way that free-speech cases sometimes do. Governor Rick Perry—perhaps still smarting from the revelation of a family hunting camp whose name included a racial slursaid the plates should be rejected. Meanwhile, the American Civil Liberties Union, while unequivocally calling the ANV flag a "symbol of racism," said the license plate should be allowed as free speech under the First Amendment.

Texas's reply is intriguing. The state argues that there's no First Amendment issue here at all—the language on the plates is its own, not the driver's. Surely the state would be justified in rejecting a license plate with a swastika. During oral arguments, Justice Ruth Bader Ginsburg asked whether "Bong Hits 4 Jesus" should be allowed on a plate, a throwback to a 2007 case. To the SCV, the state says: Get a bumper sticker. Some states do, in fact, require people to surrender their plates if they move away. But the plaintiffs point out that one problem with Texas's argument is that the state countenances several other markers of Confederate history, including "Confederate Heroes Day," making the ban seem hypocritical.

The SCV's case won the day before the Fifth U.S. Circuit Court of Appeals, which ruled against Texas. But the broader question extends past the Confederate flag to several other issues. Cristian Farias rounds up a few in The New Republic: A federal court ruled that North Carolina couldn't offer "Choose Life" plates unless they offered a pro-choice equivalent. (That decision is on hold, pending the justices' ruling in the Confederate flag case.) A New Hampshire man successfully appealed to the state supreme court after his "COPSLIE" plate was rejected under a "good taste" requirement. In 1998, man in California won the right to put "HIV POS" on his license. Decades ago, the Court ruled that New Hampshire couldn't force residents to use plates with the state slogan, "Live Free or Die."

Some diehards may still be fighting the Civil War, through license plates or other means, but that conflict was decided long ago. In Walker, the case is both less acrimonious and less long-running, but it might be nice for the justices to settle it just as definitively.