The violation, Buchwald reasoned, is not that blocking keeps the blocked from seeing Trump’s tweets—8-year-olds know how to see tweets from someone who has blocked them—but that it keeps them from replying on Twitter. That doesn’t mean Trump has to be able to see the replies, either. He can remain blissfully ignorant of his critics’ existence. But when a president speaks to the world, she held, the First Amendment preserves the right of any person to make known their dissent. In Twitter terms, it is the process of replying to his tweets—and then replying to people who reply to replies and so ad infinitum—that implicates the First Amendment right to speak to others on matters of public importance.

Buchwald’s decision makes some new law, and is sure to be appealed; indeed, she issued her opinion as a “declaratory judgment,” which is a binding statement of the law as it affects the two parties but which does not include an injunction directing any specific person to do anything, perhaps in order to avoid a messy confrontation while appeals are heard.

The decision draws on a First Amendment principle called “the public forum doctrine.” The doctrine arose in the days of the soapbox speaker, who literally needed a place to stand without being beaten up by police; 22 years ago, however, in a case called Reno v. American Civil Liberties Union, Justice John Paul Stevens created a spatial metaphor for “the vast democratic forums of the internet.” Since then, the Supreme Court has repeatedly embraced the metaphor of the Web as a huge populated place where everyone has a right to be.

What is a “public forum”? In 1939, the Court held that streets and parks are “held in trust for the use of the public” for “purposes of assembly, communicating thoughts between citizens, and discussing public questions.” That doesn’t mean that mobs can take to the streets any time they want, blocking traffic and shutting down business. Licensing and other time, place, and manner regulations are allowed, as long as they make no distinction among speakers based on their point of view. “Viewpoint discrimination” is absolutely forbidden in these “traditional public forums.”

Since then, the Court has also developed the concept of a “designated public forum”—that is, a space or an activity that has been opened up to the public as a site for free expression. These can be places—such as a municipal auditorium or concert hall—but they don’t have to be. Thus, for example, the Court has held that a university’s student-activity fee system, which subsidized various student publications, was a “designated” forum. The key is that the government has, either explicitly or by implication, invited the public to express itself in that “space.” Property-law ownership by government isn’t required; if the government uses a space or activity and opens it to expression, it can become a “designated” forum.