In most of the places where this column appears, the four-letter words it contains will not be spelled out. Instead, they will be rendered as initial letters followed by dashes.

That custom is an example of self-restraint by newspapers and Web sites that do not want to offend their readers. It is not the result of government censorship, which would violate the First Amendment.

Yet, as a case the Supreme Court recently agreed to hear illustrates, different rules apply to broadcast TV, where the Federal Communications Commission has decreed that anything it deems "indecent" may not be aired between 6 a.m. and 10 p.m. One day soon, Americans will marvel at the bureaucratic energy expended on censorship in this one arbitrarily chosen segment of the media universe.

The FCC imposed its first fine for broadcast indecency in 1975, provoked by a mid-afternoon airing of a George Carlin monologue on a New York City radio station. In upholding the fine, the Supreme Court emphasized the distinction between Carlin's "verbal shock treatment," involving the deliberately provocative, repeated use of expletives and "the isolated use of a potentially offensive word."

For the next three decades, taking its cue from the court, the FCC let stray expletives slide. Then Bono got a little carried away at the 2003 Golden Globe Awards, where he pronounced his award for best original movie song "really, really f---ing brilliant."

In response to complaints orchestrated by the Parents Television Council, the FCC's Enforcement Bureau said Bono's expletive was not indecent because it was not really a sexual reference and in any event was "fleeting and isolated." Five months later, the commission reversed this finding, along with its longstanding policy of overlooking isolated vulgarities. The FCC later ruled that expletive-containing comments by Cher at the 2002 Billboard Music Awards and by Nicole Richie at the 2003 Billboard Music Awards were indecent as well.

Last June, in response to a lawsuit by broadcasters, the U.S. Court of Appeals for the Second Circuit ruled that the FCC had violated the Administrative Procedure Act by failing to "articulate a reasoned basis for its change in policy." That decision, which the Supreme Court now has agreed to review, did not definitively address the broadcasters' constitutional objections, but the court was skeptical that they could be overcome.

The Second Circuit suggested that the FCC's indecency rules are unconstitutionally vague, creating "an undue chilling effect on free speech" by drawing seemingly arbitrary distinctions. A single "f---" or "s---" on a live awards show can cost a network millions of dollars, for example, but the same words are OK in a "bona fide news interview," even if the interview is a thinly disguised promotion for one of the network's own entertainment shows.

The accidental airing of Cher's "f--- 'em" is indecent, but the deliberate airing of the very same footage in the context of a news report is not. The "repeated and deliberate use of numerous expletives" is OK in a fictional World War II movie because they are "integral" to the film yet indecent in a documentary about real-life blues musicians.

It's obvious by now that the FCC makes up the rules for acceptable speech as it goes along. In the paradigmatic example of broadcast indecency, Carlin's monologue about "the words you couldn't say on the public airwaves," there's no question that the expletives were "integral" to the routine, which was partly about the very censorship to which it became subject.

The premise underlying the Supreme Court's decision upholding the fine for Carlin's monologue was that TV and radio over the airwaves are "uniquely pervasive" and "uniquely accessible to children." With nine out of 10 U.S. homes receiving cable or satellite TV, with downloads and DVRs making a hash of "time channeling," with ratings and parental controls available across video sources, that premise is no longer tenable. The only question is how much longer the courts will pretend otherwise.