As you'd expect, the trailer for Kevin Smith's Zack and Miri Make a Porno explains the movie's premise: Zack (Seth Rogen) and Miri (Elizabeth Banks) become adult-movie stars when they can't pay the rent. What you might not anticipate is the trailer's extreme lewdness: Justin Long name-checking a gay porn film called Glen and Garry Suck Ross' Meaty Cock or Rogen's declaring that he'd be happy to watch "a tape of Rosie O'Donnell getting fucked stupid."* Vivid descriptions of male genitalia and Rosie O'Donnell in flagrante delicto—these are the glories of the red-band trailer.

Just a few years ago, the coming attractions were a safe haven for cinematic prudes. But this year, R-rated trailers—known as "red bands" on account of the red, "Restricted Audiences Only" warning that precedes them—have become omnipresent. According to the Motion Picture Association of America, nearly 30 restricted-audience trailers have been approved so far in 2008, already matching the number accepted between 2000 and 2006. (All-audiences movie trailers, always prefaced by a green band, still run before the vast majority of cinematic fare.) In surveying the recent crop of restricted trailers, it's apparent that the studios are still adjusting to the red-band universe: The aesthetics of the R-rated trailer remain up for debate. Which naughty bits should be thrown on the screen as an enticement, and which should be held in reserve for paying customers?

As an advertising medium, the restricted-audience trailer serves a handful of valuable functions. Similar to slapping an "unrated version" tag line on a DVD, putting a red band in a front of a trailer promotes the idea that the content within is highly titillating. (And as with unrated DVDs, that advertising can be misleading; Paramount produced an R-rated trailer for the PG-13 Beowulf.) A red-band trailer, with its greater allowance for blood, can also do a better job than an all-audiences version of showing that a movie like Mirrors is slice-your-neck-open gory rather than just boogeyman-around-the-corner creepy. It's also handy when the MPAA has deemed a movie's entire reason for being inappropriate for impressionable youths. The red-band Pineapple Express preview kicks off with the line, "Goddamn, that's good weed." The bowdlerized green-band trailer starts with … coughing.

The MPAA places more restrictions on what you can and can't see in trailers than it does on the content of feature films. Restricted trailers may not show "excessive sex or violence," "dismemberment or excessive gore," or "genitalia/pubic hair," among other things. All-audiences trailers are governed by an even more draconian code, leading to such salami-hiding absurdities as a weed-movie advertisement that doesn't mention weed. Along with the ban on drug references, green bands can't include "ménage à trois, group sex or nudity of any kind … scenes containing blood or open wounds … [or] offensive language, gestures or lyrics."

Nevertheless, before the recent red-band renaissance, open-wound-and-group-sex-free green-band trailers were pretty much the only show in town. That's because the big theater chains voluntarily stopped showing red bands in 2000, after a Federal Trade Commission report, "Marketing Violent Entertainment to Children," highlighted such child endangerments as an I Know What You Did Last Summer trailer that made "verbal references to mutilations."

Red-band trailers weren't particularly common before 2000. Just as an NC-17 rating branded a feature as outré, restricted trailers in those days were mostly reserved for scandalous material like Showgirls and Madonna's Truth or Dare. Universal's R-rated preview for 1999's American Pie, for example, highlighted the film's man-on-crust sex scene, making it clear to audiences that this was a different kind of teen comedy.

The decision to ban R-rated trailers in the wake of the 2000 FTC report had more to do with public relations than with protecting kiddies. According to MPAA rules, red bands can be screened only before R or NC-17 movies; the feds were kvetching about tawdriness in all-audiences trailers that screened before PG movies. (The FTC tut-tutted, for example, the placement of green-band adverts for South Park: Bigger, Longer & Uncut before the PG-rated Star Wars: Episode I.)

Whatever the rationale for the theaters' red-band kibosh, studios mostly stopped making them once it went into effect; the R-rated trailers that did get made were seen by so few eyeballs that they might as well not have existed. Maria Pekurovskaya, senior vice president of creative advertising for Universal Pictures, says that marketing films in green has its frustrations. The green-band trailer for Road Trip, for example, left Pekurovskaya worried that they "weren't able to truly represent the film."

Universal is now back in the game, making red-band trailers for movies like Wantedand Forgetting Sarah Marshall. That's partly because a major exhibitor—Regal Entertainment Group, which operates the most movie screens of any company in the United States—decided in March to allow restricted trailers on its screens. But Regal's decision is more a symptom than a cause of the red-band renaissance. While it's still rare to see a red-band trailer in a brick-and-mortar theater, you can fire up a couple of dozen on the Web any time you want. The MPAA mandates that online red bands must be restricted to sites that cater to an adult audience or be kept behind some kind of age-verification wall. Even so, most red-band adverts are easy to find on YouTube—at least until the Federal Trade Commission decides it's time to issue another 116-page report. (The MPAA has created an in-between trailer category, the yellow band, that's designed for "age-appropriate Internet users." It hasn't really caught on yet—only six or so have been made in the last 18 months—and probably won't, considering how easy it is to find the more salacious red bands.)

Given the popularity of movie trailers on the Web, the potential audience for a red-band preview has gone from minuscule in the Showgirls era to virtually limitless in the time of Pineapple Express and Zack and Miri. Compared with an R-rated trailer that's screened in theaters, a Web-based red band is more likely to get talked up and to reach a target audience of (possibly under-17) fan boys who'll line up for a screening on opening weekend.

Along with doing heavy lifting as a viral marketing tool, red bands can also give potential viewers a better sense of what's in the movie. Pekurovskaya, who has done publicity for Judd Apatow in recent years, says the red band is the perfect medium to display the auteur's unique style. "On the Judd films, they are the juxtaposition of the really raunchy with the very sweet," she says, "and when you can only show half of that equation, you're actually misrepresenting his films. … You're doing a bit of a disservice to the audience."

The studios are still learning to deal with the relative lassitude of the restricted-trailer art form. Considering the red band's restrictions on "excessive sex or violence," trailer-makers have the most room to play with language. The red-band trailer for the Coen Brothers' Burn After Reading, for example, does little more than restore a few shits and a dickwad that were cut from the all-audiences version.

Despite those minimal alterations, that Burn After Reading trailer really works, showcasing the film's distinctive patter without using the trailer as an innuendo-enlarging funhouse mirror.

While a few extra shits never hurt anyone, there might be some value in holding stuff back. It's a long-held complaint that movie trailers give away too much—that marketers dump all the best lines into the preview, enticing people to spend $10.50 to sit through a movie that's already been strip-mined for its richest material. R-rated trailers exacerbate that problem by revealing swaths of the movie that were previously untrailerable.

Compare the red-band trailer for Superbad with the green-band one. The R-rated version includes the lines "I am truly jealous you got to suck on those tits when you were a baby," "the funny thing about my back is, is that it's located on my cock," and "I arrested a man-lady who was legally named Fuck." The all-audiences trailer has none of that material, relying on well-placed cleavage and quick, pre-swear-word cutting to suggest the movie's saltier material. The red-band trailer is probably more likely to get teenagers to buy a ticket for opening weekend; it's also a better indication of what Superbad is all about. But perhaps the movie would be more enjoyable if you'd seen less of it beforehand.

Correction, Sept. 22, 2008: Due to a copy editing error, this article misquoted a line from the trailer for Zack and Miri Make a Porno. In the trailer, Rogen's character says he would like to watch "a tape of Rosie O'Donnell getting fucked stupid." (Return  to the corrected sentence.)