There is a society, which may well resemble the real world at some time in the present or past. And then, alongside that society, there is a whole other social system with its own rules and hierarchies. It may be secret or just obscure, or even merely hard to access in some way. This is the Wainscot Society, sometimes just called a "wainscot" for short.
(Wainscots are wooden panels on the interior walls of houses; the trope name comes from the fact that the secondary society lives "behind the wainscots", sometimes literally. The Trope Namer is an entry in The Encyclopedia of Fantasy, which discusses many examples.)
The important thing about this trope is that beings living within the wainscot can interact with the mainstream society, and the physical locations of the two populations overlap; this isn't the funny foreign country next door. However, not everyone can move between the two societies, either because they just don't know that the other group exists, because only certain types of being (who may be unknown to or persecuted by the mainstream society) can operate within the secondary group, or because of physical or magical barriers. Exactly how easily it is to move between the two societies varies from case to case, but the transfer must be quick enough that it can be what starts a story; it shouldn't require a whole novel or movie just to make the shift. If it's inconceivable for more than one or two beings ever to shift between the two societies, the trope isn't present. Many wainscot fantasies involve multiple transfers in the course of the story.
It is also required that the secondary society is a fully-fledged society, with family groups and traditions; "ordinary" secret conspiracies and spy agencies don't qualify.
Muggles from mainstream society may sometimes blunder into the wainscot by accident, or people or creatures from a wainscot may occasionally enter human society, bringing strangeness with them. When discovering or entering the wainscot defines the beginning of a hero's story, it functions as The Outside World. Someone who can move unusually freely between the two societies is a Child of Two Worlds. Low-budget Wainscot Societies may have to repurpose a lot of junk from the mainstream society, so the trope sometimes adds a bit of Scavenged Punk.
Sister Trope to Masquerade (a system of intensive secrecy, and therefore one means by which a Wainscot Society in close contact with mainstream society may be kept separate), and Mouse World (a setting full of beings much smaller than normal humanity, who may well form a Wainscot Society). In other cases, the wainscot's population may be based Beneath the Earth, in an Underwater City, on an unusually convenient Island of Mystery, in a Alternate Dimension (maybe a Dark World) to which fairly accessible portals exist, or anywhere else that can somehow permit fairly easy access to the mundane world — or they may just be Invisible to Normals. A really Ancient Conspiracy may have become a fully developed society in its own right; the Alien Among Us might be one of many, who have planted a replica of their home society among humans. The Hidden Elf Village is more detached from the mainstream than this trope requires, though if its inhabitants are forced into increasing contact with the rest of the world, it may be transformed into a Wainscot Society.
- The Kings and their Clans in K operate like this, even though the system has only been in place since the end of World War II. They have their own rules, procedures, and customs that differ between the Clans, but all have similarities.
- One Piece has an Island of Mystery, Green Bit, linked to the larger island of Dressrosa by a bridge and a tunnel. The inhabitants of Dressrosa are unaware that Green Bit is inhabited by a community of dwarfish beings who are nonetheless able to visit Dressrosa quite regularly to raid it for supplies. Though the end of the arc reveals that the citizens of Dressrosa are completely aware of the dwarves and just pretend to be ignorant.
- The Eternals in the Marvel Universe are an ancient, small, but widespread group of very powerful beings who tend to depend on The Masquerade when living among humans, but who also sometimes form semi-independent communities in remote locations.
- Also in the Marvel Universe, the "Morlocks" (mostly seen in the X-Men comics) are (or were) a minor Wainscot Society of freakish-looking mutants living in tunnels under New York, but occasionally visiting the surface.
- Spirited Away has a world of spirits with complex social arrangements, generally unknown to humans but accessible to people who wander into the wrong abandoned amusement park.
- The toys in Toy Story and its sequels appear to have a society of sorts, which only operates when humans aren't looking — but they necessarily take a keen interest in human activities.
- The titular "There" of Baśń O Ludziach Stąd is one, complete with a King of the Homeless. And a superhero (he wishes).
- In The Christmas Toy, toys are alive and have their own society, but only when no humans are looking.
- The assassin world of John Wick seems to work this way. They have a completely separate economy from the rest of the world, normal people have almost a Weirdness Censor about the business the assassins undertake (several times the assassins have shootouts in public places, resulting in little reaction), authorities studiously look the other way, the infrastructure of their society seems almost Diesel Punk or Cyberpunk at times, like they live in a different genre from the rest of the world. The main place where the assassin world and the "real" world seem to overlap is, obviously, the organized crime community. The son and heir of the leader of The Mafiya has no respect for nor any clue who John Wick even is, whereas the mere description of what his son did to John Wick leaves the boss almost completely speechless. Elsewhere in the assassin world, there's also a King of the Homeless who commands an army of invisible couriers and killers.
- In The Last Witch Hunter, witches' society exists alongside humans, and they have things like their own bars, fashion shows, shops and The Beautiful Elite.
- The Matrix features two Wainscot Societies, both of which uphold The Masquerade to avoid drawing the ire of The Man:
- The Rebels of Zion who, in one reality, live in the sewers and abandoned power plants of the post-apocalyptic world and in the other regularly do business in the sewers and abandoned hotels of the slick cyberpunk world of the late 90s, early 2000s. They dress outlandishly, use weapons and martial arts with borderline supernatural ability, and dive from rooftops without a thought.
- The Renegade Programs of the Machine World, who flee from the behind-the-scenes world of the machines to live in the Matrix. Their abilities and appearances give the human world its myths of angels, werewolves, and aliens. One of the renegades, the Merovingian, commands most of their society, with the Oracle guiding the rest to avoid him as best they can. Whereas the human Rebels are distinctly Cyberpunk and Sci-Fi in their aesthetic, the renegade programs seem much more aligned with Urban Fantasy and Magical Realism.
- The Men in Black movies have alien immigrants forming a rather chaotic Wainscot Society, policed by the (mostly) human Men in Black to enforce a rather shaky Masquerade.
- Nightbreed has a dark sort of wainscot in the form of the Nightbreed and Midian.
- Many traditional stories have fairies, djinn, or similar beings living invisibly alongside humans, but occasionally becoming visible or otherwise interacting to do strange things. For example, Welsh tales told of fairy folk from an invisible island in Cardigan Bay coming ashore to trade with humanity, while Arabian legends speak of good Muslim djinn performing the pilgrimage to Mecca, invisibly.
- Magic users - Free Spirits - in Akata Witch live in a Masquerade, as is usual with magical societies involving children.
- Borribles are runaway children who have undergone an unknown process that gives them pointed ears and immortality. They live on the underside of human society: stealing what they need to survive, living in abandoned houses, and trying not to be captured by the authorities. Their enemies, the Rumbles, are fascistic rodent-like beings with a wainscot society of their own.
- Both The Borrowers and the Nomes Trilogy provide examples of Lilliputians with literal Mouse World Wainscot Societies (as they live behind the wall panels of human buildings). Both these races are quite small and scattered in the modern world, but have substantial histories; the Nomes especially have a working society in a department store. The Littles are very similar to the Borrowers, but take the Mouse World concept a step further, having some mouse-like features (i.e. a tail, buck teeth, and large ears).
- In H. P. Lovecraft's Cthulhu Mythos, various twisted cults and factions have what amounts to a parallel society in port cities and remote human communities, hidden from the mass of society but with a structure of its own. Ghouls, mostly living under human cities, have fairly frequent contact with humanity, often via such cultists and maniacs, as well as happily eating any human corpses they can get hold of, while the aquatic Deep Ones and their half-human hybrids have a fair amount of influence in the human world.
- In The Dresden Files, All Myths Are True and form multiple interacting Wainscot Societies. The Fair Folk live both in our world and the Land of Faerie, multiple types of vampires prey on humans from the shadows in different ways, and wizards and others of varying magical ability try to protect the muggles from it all. An Extra-Strength Masquerade and Weirdness Censor are required to keep those muggles from noticing anything. Interactions between these societies are governed by the Unseelie Accords, which set out rules regarding Sacred Hospitality, handling of conflicts, and other such matters.
- In the world of Fafhrd and the Gray Mouser, the rats of the great city of Lankhmar are intelligent (in at least some cases), have their own society which is in occasional contact with humanity, and sometimes meddle in the human world — making them an example of a wainscot within a big Sword & Sorcery fantasy city.
- There is something of this in Zilpha Keatley Snyder's Green-Sky Trilogy. For many centuries, the tree-dwelling Kindar have lived in peace, completely unaware of their nearest neighbors, the Erdlings — actually other Kindar who were banished down there in the early days. The Erdlings have subtly interacted with Kindar society all along. They hang out under the orchards to catch falling fruit. They rescue and adopt babies who fall out of the trees. And Kindar who ask too many questions still end up imprisoned down there. When all of this is discovered, the Erdlings become de-wainscoted and the two societies prepare to integrate.
- The "wizarding world" in the Harry Potter franchise is a particularly powerful Wainscot Society, operating with a masquerade. Although it is largely self-contained and self-reliant, and hence is almost too detached from the mainstream world to qualify as a wainscot, contact between the two worlds does continue. In particular, the wizards are too few to maintain a viable gene pool, and must interbreed with muggles to avoid dying out. Other wizards are born to muggles and must be integrated into the wizarding world, having grown up with no knowledge of it.
- The ancient gods in The Long Dark Teatime Of The Soul have their own entire otherworldly realm, but find themselves repeatedly forced to pass through ours. Unusually, traveling to the other dimension and back is easy enough that Dirk is able to develop the knack as soon as he realizes it's actually possible.
- Neverwhere is all about a magical underground Wainscot Society under modern human cities. Un Lun Dun works similarly, but as a Deconstruction.
- The Others in Sergey Lukyanenko's Night Watch (Series) mostly belong to the Night Watch and the Day Watch, two secret powerful organizations of supernatural beings in a state of cold war, permanently looking for a possibility of gaining a decisive advantage. Muggles are unaware of all this stuff, even when magical fights occur, since all supernatural activity remains in "The Twilight", a "mirror-world" of magical energy. Both sides have agreed to respect the masquerade.
- The People are psychic aliens forming a fairly small wainscot.
- The Occult London community, in the Shadow Police books, work like this. Even with the Sight, it's hard to sort out the real practitioners from the wannabes, and even harder to get a toehold in the community.
- This is a common element of the Wuxia genre — martial artists belong to a second society known as "Jianghu" where Asskicking Equals Authority. Sometimes it is taboo to involve Jianghu residents in the power struggles of the wider world or vice versa, while at other times the intersection between the heroes of the jianghu and the government of the Empire (or the Kingdoms) defines the plot.
- Beauty and the Beast concerns the denizens of a society living in the various tunnels beneath New York City, occasionally interacting with normal humans such as the heroine.
- In an episode of Bones, Booth and Brennan investigate the death of a woman who was investigating the underground denizens of Washington DC, who are depicted as forming something of a distinct society; one of the main guest stars is a vet who suffers from PTSD and who lives down there.
- Buffy the Vampire Slayer and its spin-off Angel have vampires, demons, and other supernatural beings running a society of sorts in parallel to the humans on whom they prey, mostly preserving The Masquerade.
- The wesen in Grimm are mythological creatures who live alongside humanity, but hidden from them, posing as humans..
- This is the entire point of Don't Rest Your Head. Those who lose the ability to sleep are able to find doors into a metaphorical Crapsack World — and once they return the Crapsack World is quite capable of following them back.
- In Nomine has angels and demons operating among humanity while remaining responsible to their superiors — so Heaven and Hell are the wainscots here. A Masquerade is enforced, mostly to preserve secrecy on a case-by-case basis, and to allow humans free will.
- Both the Old and the New World of Darkness feature multiple interacting hidden factions — of vampires, werewolves, wizards, faeries, etc. — who have substantial, organised social systems of their own. Vampires have their Masquerade; other beings have less formal systems of secrecy.
- The Small Folk is a game about a Mouse World Wainscot Society of magical Lilliputians.
- In Breaking Cat News there are the Robber Mice, four (or more?) female mice in Blatant Burglar black and white prison stripes and Domino Masks. They remove these on festive occasions, as you can see in BCN's November 2015 Thanksgiving sequence (updated version for the newspaper version here). Throughout the year they abscond with food and various items from the People and cats alike, using them to decorate their own living spaces.
- In the Webcomic The Creepy Case Files of Margo Maloo, the monsters live in abandoned or unused places in Echo City. Unfortunately, there is only so much space to be had, and the city is gentrifying, so The Masquerade is beginning to buckle under the strain. This is where Margo Maloo, Monster Mediator, comes in.
- Sam & Fuzzy has the second world, or the Underground in more common parlance, a semi-secret society of supernatural beings whose existence would break The Masquerade. They live Beneath the Earth in a giant network of tunnels and Underground Cities. Regular humans (and the less noticeable supernatural beings) that are aware of the Underground can move freely between the surface and Underground, though not without a considerable Masquerade created by the Ancient Conspiracy. All others are quarantined to the Underground, and offenders are fitted with an Explosive Leash or outright Killed to Uphold the Masquerade. In the Sam and Fuzzy-verse, the Underground is significantly ahead of the overworld technologically, and is often used as a testing bed for new developments that the surface world isn't ready for yet. The end of the comic's Myth Arc leads to an Unmasked World, permanently removing the figurative Wainscot.
- Skin Deep's fantastic creatures mostly live as humans using a Transformation Trinket, but have a network of safe-houses called "Avalons" where they can be themselves. Most Avalons are small, located in supposedly abandoned buildings, but some are entire self-contained towns hidden in remote areas or inside the walls of giant warehouses. The bigger ones also serve as homes for "monsters" who can't disguise themselves, or those who simply don't choose to enter human society.
- Skin Horse has many such groups, ranging from the strange beings in the Basement to a society of intelligent animals living in Cleveland.
- American Dragon: Jake Long is built upon the idea that mythical creatures live among humans in separate societies, with occasional intermingling. Jake, for instance, is an Oriental dragon on his mother's side, a fact that is kept hidden from his Muggle father.
- Men in Black features the same Wainscot Society as the live-action film, above, as it is a continuation of such.
- In My Little Pony: Friendship Is Magic, the adventures of Daring Do and her adversaries are sufficiently removed from Equestrian society that she can sell books about her adventures as works of fiction under a Pen Name.
- Rise of the Teenage Mutant Ninja Turtles: New York City is home to an entire society of mutants and mystical beings, living in the city's shadows. Most mutants have some way of hiding their appearance, and most of the places they frequent are hidden behind layers of magic. This is on top of the ancient mystic city that sits far below New York, where the mutagen and most of New York's mystic energies originated.