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.)