Answer that question and you unravel a lot of the mysteries of Brexit. The British Supreme Court is a brand-new institution, still not even a decade old. It arose from Britain’s entanglement with the Continent and was drawn up, under Prime Minister Tony Blair, partly to harmonize British law with the European Convention on Human Rights. Parliament, by contrast, is ancient enough to need no BBC sidebar, and the royal prerogative goes back centuries. Prime Minister John Major, a Tory, used prorogation as a tactic in the 1990s, as did his Labour predecessors in the 1940s.

Cases such as the one decided on Tuesday often seem less about adjudicating between quarreling parties than effecting a transfer from one constitutional framework to another. Over the centuries, Parliament has taken various monarchical prerogatives and rendered them essentially democratic, while allowing Britain’s institutions to retain, superficially, their royal pomp. The new Supreme Court is taking those democratic prerogatives and rendering them subject to judicial correction, while retaining the language of democracy. The court is an arm of the class that brought Britain into the European Union, a homegrown element of the now-repudiated European Union itself.

Such institutions are changing the whole of British politics. The country’s “unwritten constitution” consists of the laws Parliament passes and certain customs associated with them. Without a written constitution to circumscribe its powers, a court can seize on anything and interpret it in any direction.

The opening section of the Supreme Court’s decision on Tuesday insists, dubiously, that the case “arises in circumstances which have never arisen before and are unlikely ever to arise again. It is a ‘one-off.’” Such language is reminiscent of Bush v. Gore, the United States Supreme Court case that effectively declared the winner of the 2000 presidential election. As we read Tuesday’s ruling, it is worth recalling that nothing undermined public confidence in that American decision as much as the court’s insistence at one point that it not be read as a precedent.

The British court’s decision quotes a 1611 case to the effect that “the King hath no prerogative but that which the law of the land allows him.” But that assumes the court will determine what the law of the land is, not what it should be. To the frustration of the government’s defenders, the court not only judged the case at hand but also speculated about what would happen if the power of prorogation were unlimited.