Visual displays, symbolic acts, choice of attire, distribution of religious pamphlets, contributions to political campaigns, and the posting of pornography on the Internet are all eligible for First Amendment protection. In 1943, the Supreme Court ruled that requiring school students to salute the flag denied them their First Amendment rights, and free speech came to include the freedom not to speak, too [source: PBS]. Facebook "Likes" became protected speech in 2013 [source: Kelly].
Free speech, then, is actually freedom of expression. Because the First Amendment is so broad, it is ultimately U.S. courts, and particularly the Supreme Court, that must determine which expressions are and are not protected from government interference [source: Legal Information Institute].
The most widely recognized criterion for determining the constitutionality of any proposed limit comes from Justice Oliver Wendell Holmes Jr. in explaining the majority opinion in 1919's Schenck v. United States[source: Encyclopaedia Britannica]. Charles Schenck had been arrested for distributing material discouraging compliance with the World War I draft, a violation of the 1917 Espionage Act. On appeal, the U.S. Supreme Court decided that national interest in drafting soldiers trumped Schenck's right to free speech, and that the Espionage Act was constitutional. The explanation established a line of reasoning that is still in use a century later: Do "the words create a clear and present danger that they will bring about substantive evils Congress has a right to prevent?" [source: University of Missouri-Kansas City]. Holmes' opinion also included the oft-cited line: "The most stringent protection of free speech would not protect a man in falsely shouting fire in a theater and causing a panic" [sources: Bill of Rights Institute, University of Missouri-Kansas City].
Still, wartime dissenting speech has been granted protection. During the Vietnam War, a man was arrested for disturbing the peace when he walked through an L.A. courthouse wearing a jacket that read "F*%k the Draft." The court found him to be within his rights, because anyone who saw his jacket could simply look away [source: FindLaw].
Simply stated, determining limits on free speech is about weighing public versus private interests [source: Illinois First Amendment Center]. This typically happens when a person is convicted of breaking a law, and the conviction is appealed on the basis that the law in question violates the right to free speech. The court may then establish a specific limitation if it finds the speech in question doesn't qualify for protection and/or it broke a law the court finds constitutional.
In addition to national security and interests, cases involving issues like incitement of violence, libel and obscenity all have resulted in limits on the right to speak.