This story appears in the January/February 2017 issue of Playboy. Subscribe

In April 2015, Marvel released a preview cover of the first Inferno, featuring Goblin Queen Madelyne Pryor in a black leather crop top that revealed the underside of her breasts, colloquially known as underboob. The look wasn’t without controversy: By the time the issue debuted in print a month later, the character’s abbreviated top had been swapped for a more conservative full-length shirt.

Marvel’s preview cover of Inferno, featuring superhero Madelyne Pryor.

Bloggers speculated that the change was a result of Marvel trying to appease female consumers who’d expressed skepticism that the Goblin Queen would be equipped to fight crime in such a schmatta. While undeniably popular with heterosexual gentlemen, such garments have inspired scorn in women, who argue that the look has no utilitarian purpose. That’s now changing.

Despite being removed from Marvel’s cover, underboob is beginning to transition from fashion faux pas to pop culture mainstay thanks to celebrities including the Jenner sisters and Rihanna, the latter of whom is such a fan of the look that she got an underboob tattoo in 2012. In September, Kendall Jenner snapchatted herself donning an underboob-baring crop top while encouraging young people to register to vote, captioning the image “Underboob is my ting [sic].” Social media went nuts.

It’s hard to know whether the underside of Jenner’s breasts inspired millennials to head to the polls. But we do know the scientific term for what’s been called everything from “bottom cleavage” to “under-curvature of the breasts” to “Australian cleavage” (presumably because it’s down under). According to Florence Williams, author of the book Breasts: A Natural and Unnatural History and a host of the audio series Breasts Unbound, the technical name for the crease where the breast’s bottom meets the chest wall is “inframammary fold.”

“Like everything about breasts, there’s a huge variety among women in terms of whether they have one or not,” Williams explains. “Some smaller-breasted women may not have one at all.”

For centuries, the underboob has been relegated to the obscure corners of fashion history, simply because clothiers have given women little opportunity to show it off. After the corset was invented in the 16th century, fashion cared only about displaying a woman’s décolletage, neck, chest or cleavage.

Clearly, the underboob arose at least in part as a way for designers and photographers to showcase a new part of the breast that hadn’t been seen before, without verging into taboo (i.e. nipple-baring) territory. “It’s a design puzzle: What are the bits and pieces that can be shown? What are some of the new ways that fabric can be draped on the breast?” says Marjorie Jolles, director and associate professor of women’s and gender studies at Roosevelt University, who specializes in fashion. “Designers ask themselves, I can’t show a nipple, so what can I show? And what will capture our imaginations and speak to our fantasies in some way?’”

It wasn’t until the American sexual revolution of the 1970s that the underboob was brought out from the shadows with the advent of boob-baring clothing such as the crop top and the tube top. The trend was highlighted on PLAYBOY’s July 1974 cover, with model Christine Maddox in a soaking-wet cropped Playboy T-shirt.

The aerobics craze of the 1980s, which spawned the DIY aesthetic of cutoff sweatshirts and off-the-shoulder tops, further pushed the underboob into everyday life. “It had a very real moment in the 1980s, thanks to Flashdance,” says Tiffany Yannetta, shopping director of Racked.

Model Ashley Savage in a cutoff shirt on the cover of Ween’s Chocolate & Cheese, now considered one of the sexiest album covers ever made.

The most prominent pop cultural example of underboob didn’t come until almost a decade later, when the cover of Ween’s 1994 album, Chocolate and Cheese, flaunted model Ashley Savage in a cutoff top and a heavyweight wrestling belt bearing the band’s logo. According to the cover’s designer, Roger Gorman, the concept was to pay homage to the sultry, borderline soft-core-porn aesthetic of the 1970s, as well as the album art of R&B bands such as the Ohio Players. The decision to showcase Savage’s underboob was prompted by a simple question: “How much could we get away with without showing too much?”

Initially the label rejected the cover art, fearing it might be perceived as objectifying women. After much back-and-forth, a compromise was reached. The image has since entered the indie-rock pantheon, earning Chocolate and Cheese a spot on numerous lists of the sexiest album covers in history.

Since then, underboob has become synonymous with a cheesecake aesthetic that harkens back to the big-haired, voluptuous pinups of the 1970s and 1980s while simultaneously channeling the effortless look of a girlfriend lounging in your cutoff Patriots jersey. (Remember Beyoncé’s cutoff top and red-leopard-print panties on the cover of a 2013 GQ?)

In our nostalgia-loving era, there’s a reason underboob is so in vogue: It recalls a simpler time, during the 1970s and early 1980s, when women eschewed push-up bras and implants in favor of a bouncier, more natural look.

“The unharnessed, braless breast appears again and again in fashion these days,” Jolles says. “The breast isn’t so much pushed up, which requires a lot of artifice, boning and mechanics. I think underboob represents a more natural aesthetic, which maybe signals an aesthetic that is trending away from the silicone look. It’s the natural look of a breast in its droopier state.”

That’s not to say the underboob’s new popularity is liberating for women. Social media trends such as the underboob challenge, which involves putting pens underneath women’s breasts to test their perkiness, raise the question of whether our cultural taste for underboob propagates unrealistic beauty standards. “It’s really keyed into the appeal of the youthful breast,” Jolles says. “The breast that has been through a pregnancy and age doesn’t have underboob, at least not the way we think of it.”

After all, to rock an underboob in the first place, you have to have ample breasts, and they need to be taut and perky enough to create the desired half-moon-under-the-breast effect.

“I feel like it’s a feat of science, or rather, your boobs have to be super perky,” my friend Marian says. “I’d say my boobs are on the perky side of the spectrum, but even I couldn’t pull it off.”

Even those with objectively perky breasts may not pass the exacting aesthetic standards of plastic surgeons. Dr. Daniel Maman, a surgeon at 740 Park Plastic Surgery in New York City, says he considers these standards when analyzing a patient’s breast during a consultation. “If you look at the aesthetically ideal breast, you want it to have a little bit of hang, but you want to be able to see the underboob, or crease, when you look at the person from the front view,” he says. However, he adds, women have “tremendous variation” in terms of where their inframammary folds are placed; in the case of large-breasted women or women who have breast-fed, the folds may not be visible.

Williams speculates that the Jenners’ underboob-baring crop tops plus such social media trends as the underboob challenge could create anxiety for women who feel self-conscious about the size and shape of their breasts. “We tend to be in a moment when the large breast reigns supreme, and it’s only really the large breast that has the capacity to hold something in the mammary fold,” she said.

The last thing women need is another part of their anatomy to feel self-conscious about, but as long as crop tops and bralettes rule the red carpet, underboob won’t be going away anytime soon—at least not if inframammary-fold-loving dudes have anything to say about it.

From the young-single-male perspective, my friend Dashiell explains, “underboob catches the eye and shows off the perkiness and cup-ability of the boob. Like, if you’ve got handsome underboob, you probably have lovely breasts to behold firsthand. And secondhand.” My friend Miles expresses further enthusiasm: “Cleavage and overboob and nipples get a lot of the glory because of visibility and accessibility. But underboob can be very aesthetically pleasing and great to cup, like if you want to pretend your hands are a bra.” (More than a few men also told me that exposure to underboob was one of the more pleasant side benefits of performing cunnilingus.)

Halsey causes a stir at the 2016 MTV Video Music Awards performing alongside Andrew Taggart of the Chainsmokers.

Still, crop tops verge on looking like thrift-shop workout gear, and for most women, revealing underboob is more a fashion faux pas than a statement, an indicator that they should have sized up that bra or tank top. For men, however, the appeal is simple. “The nipple is still taboo,” Williams says. “Underboob allows you to keep the nipple hidden but present a suggestion of the breast as well.”

For a long time, the underboob made few public appearances, occasionally emerging on comic-book covers, in 1980s hair-metal videos or during spin class when the lady next to you wore a tragically ill-fitting sports bra.

“It’s so impractical,” says Yannetta. “Side boob is one thing, because you can be wearing a gown. Underboob requires you to have most of your torso exposed.”

Diana Tsui, senior fashion market editor for New York magazine’s The Cut, agrees. “We’re in a time when it’s not enough just to be slim; you have to be fit too. And nothing shows that more than having a toned stomach,” she says. While side boob is “more egalitarian” in the sense that a woman doesn’t necessarily need to have huge boobs and a taut stomach to rock the trend, she says, underboob “is not as forgiving—you have to have amazing abs, plus perky boobs.”