Underboobs

When a top is short enough or strategically cut to show the undersides of the breasts it's known as Underboobs. This is the standard outfit for females in loose-fitting armor in pulp fantasy. It is possibly the most impractical outfit for fighting, ever, as it leaves vital organs exposed. Particularly when the top is loose-fitting. This sort of top needs the aid of tape to stay in place, other than in drawn examples.

A type of Fan Service, naturally. Will always imply Vapor Wear and is almost always coupled with Bare Your Midriff.

Underboobage is also known as shitapai, neathage, reverse cleavage, or Australian cleavage (as in, "Down Under". Geddit?).

The underside of a breast is almost never seen in public in Real Life (seeing as how breathing wrong could risk exposure) and thus audiences aren't used to seeing it, in theory making it sexier for being unexpected and rarely shown.

Compare Absolute Cleavage, Impossibly Low Neckline, Multi Boobage and Sideboob.

Anime & Manga

 * Bleach
 * Harribel, the Hot Arrancar Espada displays this.
 * Also the former Espada, though it only appears because of her suddenly transforming from her  form to that of an   Arrancar. In contrast to Halibel, her day wear while she was an Espada was, by female Arrancar standards, an extremely modest pantsuit.
 * Lieutenant Rangiku Matsumoto's bikini during the anime filler Beach Episode #228.
 * Sei from Burst Angel.
 * Amane in episode 8 of Tenchi Muyo! GXP is full of this trope.
 * Belldandy in Ah! My Goddess - The Movie. While receiving
 * Kan'u Unchou from Ikki Tousen had her school outfit too short.
 * Lucy in Fairy Tail. A lot.
 * The outfit Theodora from Mahou Sensei Negima wears under her cloak qualifies.
 * Naru of Love Hina had this during the Pararakelse arc.
 * Yoko's bikini top in Tengen Toppa Gurren Lagann.
 * Miyo Takano in the first episode of Higurashi no Naku Koro ni Rei.
 * From Macross Frontier, one of Sheryl Nome's favorite Idol Singer outfits consists of a hat, a miniskirt (with suspenders,) boots, and a tube top.
 * Maiya Kokuchouin in GetBackers.
 * Chris Tasker from Zoids New Century has an outfit with an open front down to past her navel.
 * Kallen from Code Geass during the festival episode where she had to wear a plush costume.
 * Also, Anya, though she's so flat one would hardly notice. You can just barely see it in R2's first opening.
 * In Soul Eater when four of the Mizune sisters combine together they turn into a woman whose outfit shows some underboob. Though technically this part would be covered were it not that one of the Mizune sisters was killed.
 * Quite often in the later manga volumes of Kochikame when portraying Reiko and Maria in skimpy outfits within chapters or concept art.
 * Lyrical Nanoha
 * Arnage of Huckebein from Magical Record Lyrical Nanoha Force.
 * Sister Shante's Barrier Jacket in Magical Girl Lyrical Nanoha Vi Vid. Not exactly a very holy attire.
 * Mimete of The Witches 5 from Sailor Moon's S series.
 * Nadia of Nadia: The Secret of Blue Water tends to reveal the underside of her breasts when she adjusts her bandeau top, which naturally catches Jean's eye, which in turn triggers Nadia's Tsundere tendencies.
 * Nadia of Nadia: The Secret of Blue Water tends to reveal the underside of her breasts when she adjusts her bandeau top, which naturally catches Jean's eye, which in turn triggers Nadia's Tsundere tendencies.

Comics

 * An early cover for the comic Animal Mystic featured the heroine in an outfit with a circle cut out of the midriff to display reverse cleavage.
 * Madeline Pryor of the X-Men during her stint as the Goblyn Queen.
 * Fly Girl from DC Comics' Major Bummer series.
 * The story "Shadowjack" in The Illustrated Roger Zelazny has this with one of Vara's costumes.
 * Several of Power Girl's particularly hideous costumes had this as a feature, until she went back to the usual Boob Window.

Films -- Live Action

 * In the documentary film Playboy's Girls of Spring Break Carolyn O'Briant's underboobs are shown when she's shown competing in a tan-line contest. NSFW

Live Action TV

 * Stephen E. Witfield's book The Making of Star Trek'' mentioned how showing this was absolutely forbidden by the network censors, and scripts mentioning costumes frequently got notes back warning against showing the area below the nipple. "Perhaps the censors think moss grows there."
 * Star Trek: The Next Generation: Tasha Yar in "The Naked Now".
 * One of the Dabo girls in Deep Space Nine.
 * A Star Trek: Voyager episode features the return of two Ferengi who have been stuck in the Delta Quadrant since their last appearance. The two swindlers make their appearance with a couple of slave girls, one of which shows this trope.
 * The Orion Slave Girls of Star Trek: Enterprise.

Music

 * The album cover of Chocolate and Cheese by Ween.
 * Sometimes Meiko or Haku from Vocaloid is seen sporting one of these shirts.
 * Megpoid's extend designs.
 * The cover of Bloodhound Gang's Greatest Hits Album, Show Us Your Hits.
 * "Underboob" by Possible Oscar praises this.

Music Videos

 * The opening scene of David Lee Roth's video for California Girls has a couple of models in a store in the upper half of tank tops. When Beavis and Butthead see the video, Butt-Head remarks how he likes it when he can see "the bottom of the thingies".
 * "Don't Speak" by No Doubt features an underboob shot of singer Gwen Stefani.
 * The video of "Bad Romance" features Lady Gaga in a jeweled...thing on top, with a kind of rhinestone fishnet sheet on the rest of her upper body.

Video Games

 * The Zaulia in Brutal Legend.
 * Dizzy from Guilty Gear.
 * Christie Monteiro's outfit in Tekken 5, as she was supposed to be dressed as a Brazilian dancer. Capoeira's nickname is "The Brazilian Dance of Death", not that actual capoeira fighters dress like dancers...
 * Death from the Magical Drop series.
 * The Soul Series:
 * Psycho for Hire Tira and her... green... bodysuit... thing.
 * Also from the Soul Series, Seong Mi-Na has been sporting underboobage since III.
 * The most recent versions of Mortal Kombat have Mileena sporting this trope.
 * In WWF No Mercy, one of Terri Runnells' 4 costumes had underboobs.
 * Makoto from BlazBlue shows this when wearing her combat outfit. Mu-12 displays a bit of this too.
 * Aoi Shiro has Kaya's whose coat plays the trope straight.
 * Leisure Suit Larry 6: Cavaricchi Vuarnet's (the fitness instructor) T-shirt.
 * Poison from Final Fight. Never mind that she's actually a guy with breasts implants who may or may not have gotten a sex change operation.
 * Leslie from Arc Rise Fantasia had this in her original character design, but this was covered up in a slight revision of her outfit later in development.
 * The Sorceress of Diablo 2 has such an outfit while she is not wearing any armor.
 * Whiptail in Heavenly Sword wears a costume that shows the underside of one of her breasts.
 * Anesthesia's Flight Attendant-themed Superstar Outfit in Rumble Roses sports some.
 * In the sequel to No More Heroes, Charlie MacDonald shows up with an entire troop of cheerleaders with shirts short enough to display this very prominently.
 * Deus Ex has several (oft-repeated) female models showing underboob.
 * One of the group collages in the official art for Azure Dreams (used on the cover of the OST among others) has Nico display this.
 * Alisa Amiella in Gods Eater Burst is a case of this.
 * Hermes Gberardini from Chaos Code has a top with a cut out.

Tabletop Games

 * Several of the Malifaux miniatures, especially Taelor, Lady Hammerstrike.
 * Shadowrun:
 * The Neo-Anarchists' Guide to Real Life had a prostitute wearing a tank top with this feature.
 * Magic in the Shadows. An illustration portrays a spirit of the voudoun loa Erzulie this way.
 * Dungeons & Dragons adventure G1-3 Against The Giants. In the "Glacial Rift of the Frost Giant Jarl" section, there's an illustration of a female fighter with this odd style choice.
 * Avalanche Press' d20 supplement Aztecs: Empire of the Dying Sun. ALL of their early 2000's d20 supplements featured some gratuitous T&A slapped on top of some moderately well-researched material, but this was one of the more improbable looking covers.
 * Last Unicorn Games' Dune: Chronicles of the Imperium Core Game. An illustration of a harem on page 8 showed a slave girl with this.

Web Comics

 * Ferretina, the Weasel Queen from Girl Genius.
 * Rayne from Least I Could Do ends up having a threesome with a woman showing this trope.
 * When Dixie, of What's New with Phil and Dixie, is in her Ninja persona, her outfit includes underboobs.
 * DiDi in Ménage à 3 strip, when she had to borrow a shirt from her much smaller friend, Sandra.
 * Ultima, Radical's doppelganger from Gaming Guardians.
 * Devil Bear, Cayenne flashes a rock band.
 * Shelly of Wapsi Square doesn't wear much when playing basketball with the guys.
 * Shifters: For no good reason.

Web Original

 * After her Heroic BSOD, Fury, from the Global Guardians PBEM Universe, took to wearing a black-thong-bikini-and-halter costume. The "halter" part was little more than a strip of black cloth that showed off the underboob quite nicely.