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"'χ'note ... A most ancient letter. Some say 'kye', but the meaning is the same. Death — a letter that spells endings."
Master Xehanort, Kingdom Hearts: Birth by Sleep

Naming something "X" makes it not only cooler, but more mysterious. "X" is often associated with anonymous persons and things that have yet to be formally named.

The fact that X represents a single unknown, generic value in mathematical equations may be partly responsible for this trope and is the source of the trope name.

See also Double X and Xtreme Kool Letterz. When used to obscure the exact year of a work's established setting, see Year X.


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    Anime & Manga 
  • X/1999
  • Racer X from Speed Racer. Played straight with respect to the other characters, but inverted with regard to the audience, since the narrator won't stop reminding you just exactly who he is.
  • In Death Note, Near literally lets X be the unknown, with "L-Kira" being the face of Kira that is pretending to be L, and "X-Kira" being the one who is doing the majority of the actual killings. It's not actually his name, but Near uses it like a name, it still uses X for mystery, and it still sounds cool.
  • Mr X, the Big Bad from Tiger Mask.
  • Ixpellia of the Lyrical Nanoha franchise, also known as the then mysterious "X/Ix/Ikusu" that the Mariage was looking for during the case in StrikerS Sound Stage X.
  • Get Backers has the child prodigy MakubeX, an orphan abandoned in a bag with the name "Makube" on it. The "X" has come to represent his unknown given name.
  • As with the games, before the second generation was fully revealed, the Pokémon anime used the Code Name "Pokémon X" for Lugia. Dr. Namba also refers to Lugia by this name, despite the episode that he debuts in having come out a while after the Generation 2 games were revealed. On a related note, Pokémon Xnote  is also the name of a CD album that was released to commemorate the 10th anniversary of Pokémon.
  • Mysterious Girlfriend X, with the girlfriend in question being Mikoto Urabe.
  • The Gundam franchise, keeping with Real Life, gives prototype mobile suits model numbers that contain the letter X. More often than not, these machines are Super Prototypes — the biggest example being the original RX-78-2 Gundam.

  • John Sargent's painting Portrait of Madame X It was originally titled Portrait de Mme ***, but Sargent changed the name because he thought 'X' sounded cooler, as well as made the subject feel more archetypical as opposed to a portrait of a specific person.

    Comic Books 
  • Taken to the x-treme with X-Men. Almost everything has an X attached to it - the Man from Room X, Planet X, x-cetera.
    • "Weapon X" underwent a bit of a Retcon when the X was revealed to mean "10" in Roman numerals. In an earlier comic it was supposed to be the Greek letter Chi.
    • The leader of the X-Men, Professor X. This is even invoked in a What If? story where Magneto, instead of splitting with Xavier, decided to stay on his side and form the X-Men with him. In that case, "X" is short for "Xavier", his actual name. Once, a flashback sequence has Moira MacTaggart point out that naming the team after himself was a bit egotistical. Xavier insists that the connection never occurred to him.
  • Element X powers Metron's Moebius chair.
  • Earth X in the Marvel Universe.
  • DC had an Earth X as well before the Crisis; it was the Freedom Fighters' world, where the Axis won World War II. According to Word of God, this is because editorial nixed the idea of calling it Earth-swastika.
  • Buck Danny's nemesis is Lady X.
  • Mister X, one of Wolverine's enemies. And no, no one knows his real name. Yet.
  • Captain America and The Avengers foe Baron Zemo (the original) couldn't remove his hood because of a Freak Lab Accident involving Adhesive-X. Admit it, "X" even makes "I'm angry because this mask is glued to my face" cool.

    Films — Live-Action 
  • Science fiction/horror movie X the Unknown.
  • In The Great Escape, the resistance organisation is called the X organisation. Its leader is called Big X. Both justified and Truth in Television: Security, obviously, was an issue, and calling something the 'X' Organization would give no clues to its purpose. Same concept was used in naming the three tunnels Tom, Dick, and Harry.
  • The kaiju film The X From Outer Space. Probably called that because the monster's actual name, Guilala, is silly as hell.
  • Monster X aka Keizer Ghidorah from Godzilla: Final Wars.
  • One of the spies in Chitty Chitty Bang Bang goes by the codename of X. His superiors get confused and think they are speaking to Rex or Tex. It doesn't help when the other spy tries to clarify that it's X as in "X and bacon."
  • There are at least three films called Project X: one in 1968, on in 1987, and the 2012 comedy.
  • Low budget sci-fi clunker The Man From Planet X.
  • Star Trek III: The Search for Spock began a tradition in the franchise where experimental starships showcasing either cutting edge ship design or technology were christen with NX rather than the typical NCC. And later rechristened with NCC when it no longer was a prototype design (the Excelsior was NX-2000 when it showed up in The Search for Spock. When it showed up again in Star Trek VI: The Undiscovered Country, in-universe eight years later, the Excelsior was NCC-2000).
  • The Day Called 'X' depicts a Soviet nuclear attack on Portland taking place on an unknown date in the near future.
  • The Spy Who Loved Me. The codename of Anya Amasova is XXX. However Bond already knows who she is as a top KGB agent, so there's little point in her having a codename except as the obligatory sexually-suggestive Punny Name.
  • The first X in X: The Man with the X-Ray Eyes refers to the protagonist's name (Xavier), but also suggests the previously unknown powers unleashed by Xavier's experiment.

  • Lampshaded and then subverted in Maskerade. Agnes Nitt wishes to be called 'Perdita X Dream' on the basis that X stands for 'someone with a cool and interesting middle name'. The Lancre locals gently mock her for it, calling her 'That Agnes who wants to be called Perditax'.
  • "Project X" is a top secret government project in Atlas Shrugged, which turns out to be a horrifying weapon of mass destruction based on certain frequencies of sound wave. The X officially stands for "Xylophone", but they're definitely playing up the mystery angle.
  • Discussed Trope in The Log by Craig Charles, in which he advises teams of adventurers confronted with a villain's huge library/database to go straight to the cabinet or folder marked "X". "Guys, no Mad Scientist ever named their Evil Plan to Take Over the World 'Project W'."
  • Nero Wolfe refers to a few villains as X.
  • Billy Collins' poem "The Names" is a tribute to the victims of 9/11. He lists one victim's name for each letter of the alphabet (Ackerman, Baxter, Calabro, and so on), but when he gets to X, he says that it will stand "for the ones unfound."
  • In Darkness at Noon, Rubashov invokes this, charging the Party has lost touch with the masses, which have become once again "the great silent x of history":
    "A mathematician once said that algebra was the science for lazy people—one does not work out x, but operates with it as if one knew it. In our case, x stands for the anonymous masses , the people. Politics means operating with this x without worrying about its actual nature. Making history is to recognize x for what it stands for in the equation."
  • In The Diamond Age, the local crime boss can't tolerate how westerners mangle the pronunciation of his name, preferring they just use the first letter of its Pinyin transliteration. Hence, "Dr. X". Eventually even the local judge starts calling him that.
  • In the introduction to the 50th anniversary reprint of the Doctor Who Expanded Universe novel Prisoner of the Daleks, Trevor Baxendale says that he knew he wanted the Dalek Inquisitor-General to have a name, but until he could work out the right name, he referred to it as "Dalek X". Eventually he realised the right name was obvious: Dalek X.
  • In Mr. Twigg's Mistake, the titular mistake involves falling asleep at the control panel of a cereal plant, so that one box is filled with an entire batch's dose of the mysterious "Vitamin X". That dose appears to gift a common mole with human-level intelligence, vast growth, and amazing dexterity along the way.

    Live-Action TV 
  • The X-Files
    • The title refers to the mysterious files regarding the supernatural that the heroes investigate. It's later revealed that these files are put into the "X" drawer because there's no specific place to put them and it has the most room.
    • Mulder puts an X on the window to get in touch with his mysterious informant with no name or background (credited, appropriately, as Mr. X).
  • The name of Ted Mosby's college radio personality alias in How I Met Your Mother is "Doctor X".
  • Mutant X, in which "Mutant X" is an underground cell of mutants protecting mutants.
  • The game show What's My Line?, where a panel had to guess the occupations of the contestants, sometimes included contestants whose names were well-known but whose faces weren't, who would be introduced to the panel as "Mr. X". (For example, Jacques Cousteau was Mr. X in one episode.)
  • A CSI: NY arc had the team chasing a hitwoman they referred to as "Suspect X"; they only learn her name in the last episode she's in.
  • In the Star Trek franchise, experimental starships are given an "NX" designation until the spaceframe is approved for mass construction and an "NCC" registry.
  • In Kamen Rider Ex-Aid, the video game theme is represented by each Rider's forms having Character Levels, ranging from 1 (a weak but necessary base form) to 99 (their ultimate forms). The villainous Kamen Rider Genm eventually gained a Survival Horror-inspired Level X form; as he explains to his ally that the X doesn't just mean "ten", but also the unknown. He proves this true by gaining a massive power boost that makes him stronger than even a Level 99 Rider. Unfortunately for him, Ex-Aid ends up getting an upgrade with a Game-Breaker ability, which lets him defeat Genm by removing his ability to transform. (Ex-Aid had also received a "Level XX" form, but that just seems to stand for 20 and isn't shown to have the unknown variable that Genm's does.)
  • Ultraman X: The titular hero is called as such because of how little the humans know of him.
  • Wiseguy: Undercover agent Vinne Terranova is Hauled Before a Senate Subcommittee, but his superior Frank McPike insists that he testify behind a screen with his voice electronically altered, as he doesn't want Vinnie to blow his cover. An annoyed Vinnie goes into the bedroom, then reappears wearing a pillow case with eyeholes cut in it. "Is this how you want me to appear? Agent X?"


    Professional Wrestling 
  • Numerous masked jobbers or managers named "Mr. X", "Señor X" or "Dr. X". This wasn't always the case, as Mr. X was undefeated until the tail end of 1953 in Berlin, Dr. X was World Heavyweight Champion of Nebraska based All Star Wrestling during 1959, Gene Kiniski and Mr. X were the Canadian Tag Team Champions in Vancouver during 1963, Dr. X was World Champion in Montreal during 1971 then Karl Von Brauner and Señor X were NWA Americas Tag Team Champions in 1979. However, The Gimmick(s) became used by one too many wrestlers and became associated with losers, rather than fearsome mysterious men, though even at their height Dr/Mr X would almost always end up losing a mask vs mask/hair/title/career match before another popped up. When "Sir X" showed in the 1990s the "jobber" label became applied. It wasn't just men either, as there were various Lady X, and Madame X around. Dr. X would sometimes have a Nurse X minion as well.
  • D-Generation X. Shawn Michaels and Triple H took the label Degenerates of Generation X and used it on themselves as a badge of pride. Sort of like N-Word Privileges, perhaps. X-Pac from the same group.
  • TNA has the X Division, its official "no limits" division, as opposed to its heavyweight division, which had a minimum weight limit. It's essentially a hardcore division but not in the Garbage Wrestler sense, in that more "high spots" are to be expected sense. Despite having competitors approaching and surpassing 300 lbs, including a celebrated champion in Samoa Joe, a weight limit was eventually put in place for those who wanted to compete in the X division, temporarily turning it into a glorified cruiser weight division until it was lifted.note 
  • Short-lived wrestling promotion Wrestling Society X, which took place in a secret underground X bunker.
  • In Japanese promotions, a mystery opponent will often be labelled on the card as "X".

  • Dimension X: The title piggy-backs on the imagination of alternate dimensions, these stories were all about what unknowns are in the future.
  • "Project X", an episode of Our Miss Brooks, has Mr. Conklin develop the eponymous secret project as a way of impressing school board president Mr. Stone.
  • X Minus One: The "X" in the name carries over from Dimension X, but now implies tension and excitement, the countdown for a rocket launch! Each episode warns of "a million could-be years on a thousand may-be worlds".

    Tabletop Games 


  • Mr. X, the Boogaloo doll by Blabla Kids seen as a supporting character in Kia Sorento commercials, "was designed to be a mysterious creature from a mysterious place who served as a secret friend to his young companions" according to a Blabla founder.

    Video Games 
  • Element X in AdventureQuest (an MMORPG). Which has now been replaced by the considerably less awesome-sounding Harm and Void elements. The Guardian Dragon still claims to breathe Element X due to a healthy diet of The Powerpuff Girls, though.
  • Mega Man
  • Resident Evil 2 has Mr. X, the name given to the Tyrant that stalks you.
  • Ace Combat 7: Skies Unknown has Mister X, a nickname given to enemy ace Mihaly A. Shilage.
  • In the Avernum series, one of the archmages who govern magical use is named X. He's far more secretive and secluded than the other two.
  • In Condemned: Criminal Origins, there's "Serial Killer X," which... really doesn't make sense other than that the letter "X" sounds mysterious.
  • The X Parasites from Metroid Fusion, which are capable of consuming and mimicking any living organism, and can even become horrible mashups of every creature an individual X has previously absorbed.
  • Half-Life: Opposing Force has its "Race X" aliens.
  • Streets of Rage has the main villain, Mr. X.
  • In Freedom Force, the basis of all power is the mysterious.... Energy X! The Consonant Out Of Space?
  • Cave Story has Monster X, a Giant Space Flea from Nowhere boss.
  • The mysterious Mask X character from Punch-Out!! spin-off Arm Wrestling, who turns out to be Bald Bull once unmasked.
  • Rumble Roses features Lady X (who is later revealed to be Reiko and Fujiko's Missing Mom turned into a cyborg) as the final opponent in each of the various wrestlers' story modes.
  • Kingdom Hearts:
    • Every member of Organization XIII has a name that is a Significant Anagram of their original name with an "X" added in. Xemnas is a unique exception, as he used the name of a man whose identity he stole (Ansem). Interestingly, the Japanese syllabary has no "X", making their names words that should not be pronounceable.
    • The χ-blade, with the significance of the letter discussed in Kingdom Hearts: Birth by Sleep. To add to the confusion, it's based on the Greek letter "chi", yet pronounced the exact same way as "key", making it no sound different from "Keyblade." It can get a little awkward when using verbal communication.
    • As revealed in Kingdom Hearts 3D [Dream Drop Distance], the letter "X" is known as the Recusant's Sigil, which Xehanort used to mark the Organization's members as his vessels in his massive Grand Theft Me plan. He can even track people who wear it on their clothes, such as Sora's dream-world outfit.
    • In an example that is literal to the trope title, the Secret Reports of Kingdom Hearts III mention a mysterious girl, implied to be a character from Kingdom Hearts χ, who was used as a test subject in the experiments of Ansem the Wise's apprentices. Suffering amnesia and unable to recall her own name, she was referred to only as "Subject X".
  • Sector X is Iji is actually a subversion—although it seems mysterious at first, the actual reason it's titled Sector X is because it's the tenth sector, and X is the roman numeral for 10.
  • The villainous group in Paper Mario: The Thousand-Year Door are known as X-Nauts.
  • The X-Zone in Sonic Advance, and likewise XX in Sonic Advance 2. Both of them are the final level in their respective games... unless you're playing as Sonic and have all the Chaos Emeralds.
  • Ace Attorney - Coldkiller X (Coldkiller Z in the original)
  • Sinclair's ZX80, ZX81 and ZX Spectrum home computers. Word of God is that the original machine was named after its Z80 processor, plus X for "the mystery ingredient".
  • Pokémon
  • The Japanese title of Super Smash Bros. Brawl: Dairantō Smash Brothers X.
  • In the Romance Game X-Note, the titular Creepy Child (but not the main character) is called "X" by the researchers as a direct invocation of this trope, since he refuses to give his name and does actually represent the unknown. He turns out to be Anon. The sequel Area X follows this tradition, taking its title from the dimension Area-X, aka the 'unknown' dimension.
  • In Persona 4, Chie and Yukiko made a curry-like dish so inedible that there's no way to call it curry. Yosuke dubs it Concoction X or Mystery Food X
  • DanceDanceRevolution X and X2.
  • In Psychic Storm, "Stage X" ensues when the player encounters an unidentified Giant Space Flea from Nowhere.
  • The Doom mod Scythe 2 includes a level named "Mr. X", where the player fights his own Evil Counterpart.
  • Fate/Grand Order:
    • Saber Arturia has alternate versions of herself with strange origins and motives. An Assassin called Mysterious Heroine X, a Berserker called Mysterious Heroine X Alter, a Foreigner called Mysterious Heroine XX, and another Foreigner called Mysterious Idol X Alter.
    • There is also an Avenger called Mysterious Ranmaru X, a female counterpart of Mori Ranmaru from the same place as the Heroine Xes.
  • Deathwish Enforcers has the main villain, Big Boss X.

    Web Animation 

  • Pokémon-X. The title has been hinted as being a reference to a creature or an item.
  • In Grrl Power, a shadowy secret agent introduces himself as "X".
    Sydney: Wow that's forgettably generic.
    "X": It's supposed to be.

    Web Original 
  • Labtech X from Gaia Online.
  • On 4chan, /x/ is the board dealing with topics of the paranormal.
  • Sonic.exe and its associated works, discarding the X in the file extension name, would eventually reveal the name of the deity capturing the souls of humans to be 'X' as well.

    Web Videos 
  • Bum Reviews: Chester A. Bum explains that he doesn't want to spoil the celebrity cameo from Zombieland, and so dubs him Mr. X for a short while.
  • When a mystery guest commentator shows up during pokecapn's Sonic Unleashed marathon, medibot terms him "X".

    Western Animation 
  • Lampshaded in Sheep in the Big City, where General Specific has issues with the name of their latest hire, "X Agent":
    General Specific: I don't want an "Ex-Agent"! I want a current agent!
    Private Public: The "X" is put in to indicate mysteriousness rather than his job status sir.
    General Specific: Well, why can't he choose another letter? Like "K"?
    Private Public: "K" isn't as mysterious a letter as "X", sir.
    General Specific: What about "L"? "L" is mysterious! [waving fingers] Llllllllllllll...
    Narrator: While the General re-acquaints himself with the alphabet...
  • Red X from Teen Titans (2003). It was initially a phony supervillain identity Robin used in an attempt to get close to Slade, but someone else came along and stole the gear for himself. His identity is a total mystery.
  • Chemical X from The Powerpuff Girls, which is a chemical that Professor Utonium accidentally spilled into the formula when creating the Powerpuff Girls. This is a Bowdlerisation, since it was originally a "Can of Whoopass".
    • Likewise, the effects of the chemical can be reversed with Antidote X.
  • Planet X (see below for details) appears in many old sci-fi works. Today, most people recognize it from Duck Dodgers in the 24˝th Century, which parodies this by having it as an actual planet with a giant "X" on it, which you can find by following a trail of planets similarly labelled with other letters of the alphabet.
  • The Simpsons: Homer Simpson's online alias is Mr. X.
    • "So, Mr. X - if that is your real name..."
      • ' Mr X? Do I dare cross the final frontier?'
  • David Xanatos from Gargoyles. (yes, that Xanatos.) Sometimes he's even addressed as "Mr. X."
  • Protoform X (otherwise known as the murderous Rampage) from Beast Wars, the result of a top secret science experiment that turned out to be an uncontrollable monster. After his capture, he was loaded aboard the Axalon alongside other protoforms, under conditions of utmost secrecy, so he could be quietly disposed of somewhere far from home.
  • The X's uses this trope.
  • Dimension X from Teenage Mutant Ninja Turtles (1987).
  • Alien X, originally presented as the mysterious tenth alien of Ben 10: Alien Force.

    Real Life 
  • The variable "X" is almost always used as an unknown in mathematics starting with algebra, by convention. You'll get more of the alphabet in problems with many unknowns, but "x" is pretty much always the first one. Incidentally, it is because of this that students are eventually taught to stop using "x" as a multiplication symbol, instead opting for an asterisk or a dot, in what is most closely describable as a strange variation of the One-Mario Limit.
  • X-rays. They were discovered by Wilhelm Conrad Röntgen, but getting an X-ray sounds a lot snappier than getting your arm Röntgen'd, which you can actually do in Germany. Given that "Röntgen ray" is in line with a very long and rich tradition of naming inventions after the inventor, and that Röntgen himself (who was a German) didn't want them to be named after him, it is highly interesting that German-speaking nations and some others like Japan and Finland followed the tradition, while English-speaking nations followed Röntgen's own suggestion.
    • Fun fact: according to the Other Wiki, Röntgen called them "X-rays" because they were an unknown form of radiation at the time. Now that X-rays are well understood, calling them that is a bit of a misnomer, but the name stays per the Rule of Cool.
  • The X-Prizes are something like this, originally the "X" being because a sponsor hadn't been found, but once people realized how cool the "X" sounded, it stayed, with the sponsor's name tacked on in front of it: "Google Lunar X-Prize", "Ansari X-Prize", etc.
  • Designer X, known also as Greg Costikyan, RPG designer.
  • "Planet X" was the generic placeholder name for "whatever it is out there beyond the orbit of Neptune that's messing with its orbit". Pluto's discovery meant that "Planet X" now had the double meaning of "...beyond the orbit of Pluto" AND "the tenth planet". Of course, Science Marches On, and now not only have we discovered the Kuiper Belt and the Oort cloud, but an entire class of Pluto-sized "dwarf planets". For the record, the supposed irregularities in the orbits of Neptune (and also Uranus) were actually caused by incorrect calculations of Neptune's mass.
  • Whenever the US Air Force builds a new aircraft that's still in the experimental stages, there's almost always always an "X" in its designation. When it is through the experimental stages and ready to be put into production, the "X" is taken out and replaced with a classifying letter (e.g. the Lockheed Martin X-35 became the F-35 Lightning II). The other branches do the same with their experimental weapons systems, though slightly differently (the classifying letter is usually part of its name from the beginning; for instance, the Heckler & Koch XM 8 would have become just the M8, had it been adopted, just as the XM4 eventually became the M4). This association is so pronounced that experimental aircraft (especially ones designed for the US military) are often known simply as "X-planes." The Air Force used to do that too, such as in the case of the XB-52 (and the Army Air Force before that, such as with the XP-51).
    • Note that the X- strictly speaking applies to experimental aircraft. Once they are on the path to finalizing the design for production, it is common to switch to a Y-prefix. Compare the Lockheed XP-38, a one-of-a-kind prototype built to see what the design could do, and the YP-38, a pair of pre-production models built using lessons learned from the original.
    • 'X + number' alone denotes the experimental aircraft, but prototypes are designated in the 'XF, XB + number' style, the second letter designating purpose like fighter, bomber etc., then moving to the 'YF + number' style, when the design is in the pre-production phase. The F-35 is something of an odd number out, as instead of assigning a new number in the fighters sequence to it (e.g. F-24, following on from the adopted F-22 and rejected YF-23), it retained its '35' numerical designation, with the original prefix 'X' just changed to 'F'.
  • The Ecole Polytechnique, which is arguably the most prestigious French engineering school (college), is nicknamed "the X".
  • This is standard practice among the Nation of Islam: upon conversion, one replaces their last name with "X" on the reasoning that the former is a "slave name" (given to slaves and their descendants by slave owners), and "X" represents the unknown name lost in time. This was most notably exemplified by Malcolm X, originally born "Malcolm Little"; after he converted again, this time to Sunni Islam, he changed his name to El-Hajj Malik El-Shabazz, as taking an Arabic name is very common among converts to mainstream sects of Islam, but he remains best known as Malcolm X.
  • Microsoft's (mostly) video game API DirectX. X is taken as the variable for the APIs within the API (Direct2D, Direct3D, DirectMusic, DirectInput, etc.)
  • The XYZ Affair.
  • Before Pliosaurus funkei was formally described in 2012 it was referred to in the paleontological community as Predator X. As TV Tropes' Twitter put it: "Everyone, including paleontologists likes the letter 'X'."
  • The Åland Islands were given the ISO country code AX, because all the fitting two letter codes starting with A, such as AL and AD, were already taken.
  • An article in the April 11, 1953 edition of Billboard was about a new subsidiary label that RCA Victor was establishing. Since the name of the label was not decided on yet, the article referred to it as Label "X". It seemed to have inspired RCA: the label was officially named "X" a few weeks later and kept that name for a few years before changing its name to Vik.