X-rated: What is so special about the letter 'X'

Dangerous, mysterious and forbidden, the alphabet's 24th letter has wielded a strange and thrilling power for thousands of years. Culture and technology wouldn't be the same without it. And now both Gordon Brown and David Cameron have recruited it to the party political cause. So what exactly gives 'X' its X-factor? Jonathon Green opens up the X-files

The letter X is 24th in the alphabet and is thus, for lexicographers trudging through all 26, a welcome sign of pre-penultimacy: only two to go until the end. But X is not just any old letter, no mundane B, overextended S or unavoidable E.

X holds a mystique. It smacks of the strange, the alien and the exotic - and of the forbidden. It may be simply that this reflects its relative invisibility in the dictionary. Times have changed since, in his own great work, Dr Johnson was able to dismiss X as "a letter which, though found in Saxon words, begins no word in the English language", and allowed no entry other than that.

But initial X is still hardly a major linguistic player. My own slang database reports a mere 30 terms at this letter, whereas the monstrous S offers 12,000, and B scores 9,850. Such proportions will be found in the standard English lexica, too. Or is it the sheer unpronounceability? X-words tend to the tongue-twisting, and we steer clear.

But the letter serves a purpose. Whether implying exoticism, taboo, or technological progress, X is still capable of giving us pause. Hard-wired to acknowledge the letter as an exception, we experience it as a small but arresting exclamation. So rare in "average" words, other than in the prefix ex- (itself implying an absence, a vanishing), its appearance makes us jump, however briefly.

And, as followers of politics may have noticed, X seems to be enjoying a new variation on its themes, with its use on successive days by prime minister-in-waiting Gordon Brown and prime minister-in-waiting (somewhat longer) David Cameron. Brown extolled his vision of an "X Factor Britain", invoking the television talent show, and by which he meant the pursuit of aspiration: how anyone, as he put it, can achieve anything. Cameron, opting for another column in the populist manifesto, coined the idea of "X-listing", which seems to be the aesthetic antonym to the listing system of preserving valuable buildings, with his plans to demolish what mass opinion (informed or merely polled) deems to be architectural eyesores.

The themes are not, however, new. Brown's X suggests the undefinable, the "certain something", the "unknown quantity" of mathematics; Cameron's echoes another X, that of the censor's blue pencil or editing scissors. New Labour's X cannot be stated, New Toryism's should not be seen.

The start of X itself, like others of our letters, lies in Greek, in this case in Western Greek; that is, Greek as spoken in Naples, whence it moved up the Italian coast to Etruscan and thereafter to Latin, where it was the 21st and last letter of the Roman alphabet. In all cases, it represented the sound "ks" with a hard and noticeable "k", typically in such proper names as Xantippe, Xerxes and Xenephon.

The idea that X didn't work at the start of words was already in place. While Greeks might well have had words beginning in ks, the Etruscans and their Roman successors did not. In Latin, and in English words that developed from Latin roots, X requires a vowel. Only X meaning 10 stands out, and that was pronounced "decem". (In the United States in around 1890, X and XX meant $10 and $20 respectively, and thus the slang "sawbuck", properly an X-shaped frame or sawhorse, also means $10. The States also offers the mysterious "Ped Xing".) Such words as do start with the letter, such as xenophobia or xylophone, are from Greek, and the X is pronounced more as a Z.

It is Greek, none the less, that brought the use of X as an abbreviation for Christ. In this case, the character is not the "ks" sound but "chai", and pronounced "ch", as in Christ.

X stands at the head of such compounds as xanth, meaning yellow; xeno, meaning foreign; xero, meaning dry; xiph, denoting a sword; and xylo, meaning wooden. Such forms, with their compounds, make for a high proportion of the dictionary's still tiny list of X-entries, but let us be honest: are such words as xanthomelanous (having black hair and yellow/olive skin), xenurine (a species of armadillo), xerotripsis (dry friction), xiphophyllus (with sword-shaped leaves) or xyloidine (an explosive similar to gun cotton) tripping off the popular tongue? As an initial letter it is less than easy for English-speaking tongues to get around.

It may be coincidence that xenophobia, fear of foreigners, is one of English's few X-words, but it is also conveniently symbolic. For X, until relatively recently, has almost invariably been allied to difference, even danger. X is the scrawled signature of the illiterate, and if - scrawled on a mysterious map - it "marks the spot", who was to say what else lurked there? For 18th-century and Victorian criminals, X equalled "the cross", and the cross meant anything - and anyone - dishonest; is he "square" or "on the X"?

In 1860 or so, the police secured a violent arrestee "on the letter X". A contemporary explained that "two constables firmly grasp the collar with one hand, the captive's arm being drawn down and the hand forced backwards over the holding arms. In this position, the prisoner's arm is more easily broken than extricated."

In the world of Nancy Mitford, if not of Irving Welsh, X can mean another type of cross: angry. Though, in a gentler mode, X remains the only shorthand for kiss.

Trawling the online movie databases, one finds eight films titled simply X. One is publicised: "Suddenly he could see through clothes, flesh... and walls!", the X being X-rays; another explains how, "unable to support himself as a writer, failed author Dong-shik starts working as a male prostitute"; yet another determines the fate of the earth when the characters "cross over into the X Manga". None of it is exactly family fodder.

Neither was Brand X, the hapless alternative to whatever product was being peddled by the TV hucksters of the commercial break. That said, at least in mid-19th century slang, "double-X" could mean superlatively good, playing not, as one might have thought, on excellent, but on the racetrack jargon double-X; the horse most likely to win, and thus the optimum bet. America's V and X store offered a positively classical take on such corner emporia better known as the "five and dime", the V and X meaning five and 10 in Latin.

Slang being slang, one cannot avoid drugs, vitamin X being yet another name for MDMA and playing on the best known of its nicknames, Ecstasy. Liquid X can be gamma hydroxybutyrate (usually GHB). X, 50 years ago, meant a narcotic injection. Still, in the States, to have the X on someone is to place them at a disadvantage, while here the rhyming slang X-Files means piles.

X has always played an important part in maths and science. To quote the Oxford English Dictionary: "In Algebra and Higher Mathematics, used as the symbol for an unknown or variable quantity (or for the first of such quantities, the others being denoted by y, z, etc); spec. in analytical geometry, the sign for an abscissa, or quantity measured along the principal axis of co-ordinates." The usage was pioneered in 1637 by Descartes in La Géometrie. He pondered Y and Z for a similar role, but X won that day and all that have followed.

Similar technical uses can be found in genetics (the X chromosome), in linguistics (the X-question) and in radar (the X-band). Gordon Brown's reference to the "X factor" doubtless looked no further than the television show, but the term is older, referring to those aspects of a serviceman or woman's life that have no equivalent on civvy street (primarily, one would like to hope, getting shot at) and for which one is paid a bonus accordingly. Older still is the Navy's X-catcher or X-chaser, a pre-First World War term for a naval officer proficient in examinations or especially good at his work. And the military always liked to throw an X at their latest hardware, typically such mach-smashing aircraft as the X-1 or X-15. Slightly slower, but still capitalising on the 24th letter, was Jaguar's XK-120 sports car, in which the X, as it did in the rocket planes, stood for experimental.

It is, no doubt, the sci-fi attributes of X that have made it so popular with the rise of computers and the dot.com firms that capitalise on them: ImageX, eBenX, Xpedior, Xcare, Exult, Webex - and others). Only Q and Z and the ever-popular lower-case "i" seem to offer a similar mystique. The earliest example of the sc-fi X was probably the X-ray, discovered and named in 1895, which as early as 1919 gave rise to the X-ray dress; not exactly see-through but definitely translucent. Decades later came the New York women - skinny and sophisticated - whom Tom Wolfe defined as "social X-rays".

Today, we have the Xbox, and Microsoft's XP operating system. Desktop publishers will know QuarkXPress, while TV fans will hardly have missed The X-Files, nor readers of Douglas Coupland his 1991 novel Generation X. It is hard to believe with such spellings that such big-time products didn't have some influence on txt msgs, even if Z (warez, boiz, the "iz" found in rap-related slang) is equally influential.

Of all the Xs, perhaps the best-recalled remains the X film, a British classification created in 1951 to cover "adult" films. X, here, stood for extreme, but it could also have stood for excluded: no one under 18 was (supposedly) allowed in. But it is gone, replaced by the anodyne 18. In any case, a single X is no longer enough; three, if not four, seems to be the rule on the internet's infinity of porn sites.

X. So small, so rare, so vital, so exegetical, so extraordinary, so exemplary. Do we, as they say Down Under, give a Four-X? Well, of course we XXXXing do!

Jonathon Green is a lexicographer of slang; his most recent book is 'Cassell's Dictionary of Slang' (revised and expanded 2005)

X is for...

X Factor: Long synonymous with star quality, but nowadays most closely associated with ITV's musical talent show. While the series has propelled pop impresario Simon Cowell to international celebrity (and turned him into one of the richest men in show business), the same cannot be said for most of the show's contestants, for whom the fickle finger of fame has proved rather more, erm, fickle. Pop pickers may be surprised to know that the phrase originally meant "that part of a serviceman's or servicewoman's pay intended as a compensation for the disruptions and disadvantages of life in the armed forces".

X-Prize: Similar name, but an altogether more high-powered format. Modelled on the early 20th-century aviation prizes and aiming to spur the development of low-cost space travel, The X-Prize Foundation offered $10m to the first non-government organisation to launch a reusable manned spacecraft into space. The prize was eventually claimed on 4 October 2004 after a successful test flight by scientist Burt Rutan's Spaceship One. Rutan has since joined Sir Richard Branson's Virgin Galactic space tourism project.

X-Plane: Classification of experimental United States military aircraft. Despite their clandestine construction, X-Planes often become well known. The first, the Bell X-1, is perhaps the most notable, having become famous in 1947 as the first aircraft to break the sound barrier. More recently, X prefixes were added to stealth bombers during their development stages.

X-Men: The 2000 film version of Stan Lee and Jack Kirby's best-selling comic-book creation led fans on an exciting foray into the legendary Marvel universe. With a cast of mutants led by Hugh Jackman, Halle Berry and Sir Ian McKellen (as well as Patrick Stewart in the X-tastic role of Professor Charles Xavier) the film was a huge box-office hit, inevitably spawning a sequel in 2003, the imaginatively titled X-Men 2.

Generation X (1): Term widely used to describe those born following the peak of the post-Second World War Baby Boom. First coined by Jane Deverson in 1964, as part of a study of British youth commissioned by Women's Own magazine. Deverson's findings were deemed too shocking for the publication, as teenagers admitted that they "sleep together before they are married, don't believe in God, dislike the Queen and don't respect their parents", but she published them in book form, to predictable controversy.

Generation X (2): Also the name of a 1976 punk rock band, fronted by Billy Idol. Initially called Chelsea, the band was re-christened after Idol stumbled on his mother's copy of Jane Deverson's book. The band's best-known hit "Valley of the Dolls" (1979) was taken from the title of a cult novel by the American author Jacqueline Susann.

Generation X (3): Subtitled "Tales for an Accelerated Culture", Douglas Coupland's seminal 1991 novel captured the angst and alienation of those who reached adulthood in the 1980s. The novel portrays the economically bleak and emotionally taut lives of three friends who try to escape the overly commercialised world by living simply in the California desert. The popularity of the book brought the phrase back into common usage.

Brand X: Phrase used by advertisers (particularly in the US) to describe a competing brand or product which, while not named, is implied to be of inferior quality.

Project X: Shhh... It's a secret.

Planet X: Astronomers have long used X as the name of a hypothetical planet in the outer reaches of our solar system, just beyond the orbit of Pluto.

X-Wing Fighter: Or Incom T65 X-Wing, to give it its full name. Those who grew up brandishing a light sabre and idolising Luke Skywalker will remember the X-Wing, a starfighter class spaceship used by the Galactic Alliance and the Jedi Order in the Star Wars universe. The X-Wing was originally designed by Incom Corporation for the Empire, but the entire engineering team defected to the Rebel Alliance with the prototypes. It is directly descended from the old Z-95 Headhunter, and shares design features with the Clone Wars-era ARC-170. It is armed with four laser cannons, as well as missile launchers and deflector shielding.

Xmas: Common abbreviation of the word Christmas. The "mas" part comes from the Anglo-Saxon for festival or religious event, "Christemasse". This abbreviation is widely accepted, yet, while some see it as a useful short form, others believe it to be demeaning to Christ, representing the secularisation and commercialisation of Christmas. However, the word Christ itself has been abbreviated for about 1,000 years - long before the modern Xmas was used. A peek at the Anglo-Saxon Chronicle reveals that Christ was often written as "XP" or "XT" as far back as AD1021.

X-certificate: This classification was issued by the British Board of Film Censors in 1951, following the Wheare Report on film censorship, to signify that a film was "suitable for those aged 16 and over". In 1970, the X-certificate was redefined as meaning "suitable for those aged 18 and over", before being replaced by the "18" certificate in 1982.

Xbox: Microsoft's games console, launched in North America on 15 November 2001. Notable games titles include Halo: Combat Evolved, Amped, Dead or Alive 3, Project Gotham Racing, and Oddworld: Munch's Oddysee. Microsoft had sold 24 million consoles by June 2006. Although ahead of the Nintendo GameCube's 21 million, this is far behind the Sony PlayStation 2's 106.23 million.

OS X: Microsoft is not the only technology firm to latch on to the mysterious power of X, and when Apple launched its innovative new Macintosh operating system in 1999, OS X was born.

Features such as memory protection and pre-emptive multi-tasking made OS X much more reliable than previous Apple systems, reflecting a significant development.

X-rays: These are a type of electromagnetic radiation, widely used by doctors for diagnostic purposes. The part of the patient to be X-rayed is placed between the X-ray source and a photographic receptor to produce a shadow image of the internal structure of a particular part of the body. The X-rays are blocked by dense tissues such as bone and pass through soft tissues. Those areas where the X-rays strike the photographic receptor turn black when the image is developed.

X-Ray Spex: This UK art punk-rock band formed in 1976 and broke up in 1979, releasing memorable singles such as "Oh Bondage, Up Yours", "Identity", and one album, Germ Free Adolescents. The band featured singer Poly Styrene (born Marion Elliot) on vocals, Jak Airport (Jack Stafford) on guitars, Paul Dean on bass, Paul "BP" Hurding on drums and Lora Logic (born Susan Whitby) on saxophone. Styrene's voice was variously described as "effervescently discordant" and "powerful enough to drill holes through sheet metal".

X chromosome: One of two sex-determining chromosomes in humans. Each human being normally has one pair of sex chromosomes in each cell. Females have two X chromosomes, while males have one X and one Y chromosome.

X-Files: Science-fiction TV series created by Chris Carter, starring David Duchovny and Gillian Anderson as FBI agents Mulder and Scully, whose paranormal investigations generated a cult following in the 1990s. Its main characters and slogans ("The Truth Is Out There", "Trust No One", "Deny Everything", "I Want To Believe") became pop-culture touchstones through the decade, simultaneously tapping into and inspiring a plethora of conspiracy theories, paranoia about the US government, and belief in the existence of extraterrestrial life. It was also one of the first TV series whose fans disseminated information and trivia about the show via the internet.

xXx: Car chases, gadgets and explosions are the order of the day in this 2002 action movie. Vin Diesel plays Xander Cage, an extreme sports star who gets in trouble with the law, is blackmailed by Samuel L Jackson's FBI agent, and forced to infiltrate a Russian crime ring. Diesel's character was named after a German agent called "Triple X", seized just before America joined the First World War as he was attempting to plant bombs in American ports.

XXXX: Brand of Australian beer, produced by the Brisbane Brewers Castlemaine Perkins and known as "Four-X". This was later used as a humorous euphemism for a four-letter word, with advertisements proclaiming: "I couldn't give a XXXX". The name, introduced some time before 1878, is a throwback to the tradition of using Xs to indicate the quality of an ale. The most widely known Castlemaine beer outside Australia is XXXX Export Lager. It contains 4.8 per cent alcohol by volume (ABV) and is described as bland but palatable with a bitter aftertaste.

X marks the spot: Refers to the marking of a location on a map (a pirate's treasure map, say) or the scene of a crime in a plan or photograph.

Malcolm X: The African-American activist Malcolm Little changed his surname to X in 1952 to signify that his original name had been lost in slavery. A convert to Islam, Malcolm famously opposed the peaceful aims of the civil rights movement led by Martin Luther King, rejecting ideas of unity and non-violent protest. A film of his life was directed by Spike Lee in 1992, with Denzel Washington in the title role.

Xzibit: In 1996, the Detroit-born rapper Alvin Nathaniel Joiner IV ditched his impressive name in favour of the catchier Xzibit and promptly released his debut album At the Speed of Life. Collaborated with Dr Dre on the hit single "Bitch Please". His most recent album, Full Circle, was released last month.

XS, XL, XXL etc: In clothes sizing, the letter X is used as an abbreviation for "extra". Waistlines are expanding, so there are more Xs than ever.

X: In Kabbalistic philosophy, the branch of Jewish mysticism championed by Madonna, the letter X refers to both birth and death - perhaps explaining the singer's constant reinventions.

X: In Roman numerals, X denotes the number 10; with a horizontal line drawn above it, it means 10,000.

X: Commonly used to indicate one's choice of candidate in elections to democratic office. In the US, where voting in Congressional elections takes place today, many ballots are cast using computerised machinery. And who can forget the Florida "hanging chads" fiasco of the 2000 presidential election?

X: Also known as "The Man With the X-Ray Eyes", this 1963 film quickly became a science-fiction classic. Directed by Roger Corman, the film stars Ray Milland as Dr James Xavier, a mad scientist who develops eyedrops that increase the range of human vision, allowing one to see beyond the "visible" spectrum into the ultraviolet and X-ray wavelengths beyond. When the film was released, the visual effects used to depict these superhuman powers were spectacular.

X: The letter X torments students of mathematics at all levels, from the common multiplication symbol in arithmetic to the more complicated X of algebraic equations.

xxx: Three little kisses. Don't put them in your e-mails, though; anti-porn filtering software may prevent your fond wishes being received.

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