Constitutionally protected speech
All speech is considered constitutionally protected unless it falls within several limited exceptions. The right of protected speech is derived from the first amendment of the U.S. Constitution that reads, "Congress shall make no law . . . abridging the freedom of speech."
Under common law the U.S. Supreme Court has limited this right by deeming certain types of speech to be outside this protection. They are for the most part: incitement, obscenity, fighting words and offensive speech, and threats. Further, the Court has upheld laws that reasonably restrict speech on the basis of its time, place and manner. There is for the most part, no black letter law as to exactly what speech is protected and what speech crosses the line. The determination is always a matter of context and the specific facts of the situation.
Speech that is designed to incite violence is not considered protected speech. The Court in Brandenburg v. Ohio  clarified this when they held that only speech that is "directed to inciting or producing imminent lawless action and is likely to incite or produce such action." In that case the court overturned the conviction of a KKK leader who espoused political reform through violence. The court held that his speech was not likely to incite imminent action and thus was protected.
For speech to be considered obscene and thus unprotected it must meet the three elements of the Miller test. Under the Miller test only works that depict or describe sexual conduct can be considered obscene, and further any state or federal action must be limited to, "works which, taken as a whole, appeal to the prurient interest in sex, which portray sexual conduct in a patently offensive way, and which, taken as a whole, do not have serious literary, artistic, political, or scientific value." Further the court has defined prurient interest to mean "a shameful or morbid interest in nudity, sex, or excretion, which goes substantially beyond customary limits of candor"
Fighting words and offensive speech
Fighting words are just like they sound, in Chaplinsky v. New Hampshirethe Court held that words that are personally abusive to an ordinary citizen and that tend to incite an immediate breach of the peace are fighting words and are not protected speech.
Threats are another form of unprotected speech. The Court has held that threats of violence, that are designed to place a person or group at risk of bodily harm or death are not protected speech. However, not all threats are considered unprotected speech. The Court has held that if a reasonable person would not believe the threat to be actionable then the statement does not count as a threat. In addition, threats of social ostracism or politically motivated boycotts are also constitutionally protected speech.
Restrictions: Time, place and manner
The government is allowed to restrict speech if it does so based on the time place and/or manner in which that speech takes place. An example of this is a law prohibiting protests in residential areas before 7 a.m. and after 8 p.m. This restriction does not limit the ability of a person or group to espouse a message but does prevent loud noises around other people's houses during times when quiet would be expected. When a restriction crosses the line from regulating only the time, place or manner to restricting specific content of speech, then that restriction is unconstitutional and such speech would be protected.
- ↑ Brandenburg v. Ohio, 395 U.S. 444 (1969) (full-text).
- ↑ Id. at 447.
- ↑ This test gets its name from Miller v. California, 413 U.S. 15 (1973) (full-text).
- ↑ Id. at 24.
- ↑ Id. at 37 n.1.
- ↑ Chaplinsky v. New Hampshire, 315 U.S. 568 (1942) (full-text).
- ↑ Virginia v. Black, 538 U.S. 343, 360 (2003) (full-text).
- ↑ Watts v. United States, 394 U.S. 705 (1969) (full-text).
- ↑ NAACP v. Claiborne Hardware Co., 458 U.S. 886 (1982) (full-text).