Yesterday, Apple unveiled the iPhone 8, iPhone 8 Plus, and iPhone X, which the company is pronouncing “iPhone Ten.” Ten years ago, Apple transformed the smartphone landscape—and our very lives, for better or worse—with its first very iPhone, hence the Roman numeral X, or 10, in its new mobile moniker. Let’s take a quick look at the history of *X*, from its birth as a letter and numeral to its spread into everything from Jesus to algebra.

**The Roman Numeral X**

As noted, X corresponds to the number 10 in Roman numerals. We know the ancient Romans adapted their notational system from the Etruscans, but the deeper roots of the symbols themselves are obscure.

One theory thinks the numerals evolved out of basic tally marks. The Roman numeral I (1) was a simple notch, with every fifth one double-notched, yielding V (5) and every tenth crossed into an X (10).

Another theory thinks the numerals emerged from hand-counting. I represents one finger and V a full hand. If you hold out your hand, you can see how the gap between the thumb and index finger might resemble a V. Hold out two hands for 10 and you can imagine how two V’s, stacked on top of each other, can look like an X.

Whatever their origin, the form of the symbols were adapted to the existing letters in the Latin alphabet—which included letter X.

**The Letter X**

The ancient Greeks added letter X—called *chi*, its 22nd letter and our 24th—when they borrowed the Phoenician alphabet. Originally, X represented a *kh *sound, but certain regional alphabets took to using the symbol as a convenient shorthand for frequent *ks *combinations in the language. The Etruscans also adopted this convention—and later the Romans, which ultimately made its way into the sound and shape of English’s own X.

**The Christ X**

The Greek *chi* is the first letter in *Χριστός* (*Khristos*), source of *Christ*. This epithet literally means “anointed,” a translation of the Semitic *messiah*; rubbing oil on heads of kings, priests, and other important figures was an ancient custom used to consecrate them.

The second letter *Χριστός *is *rho—*ρ and source of *r—*and* *together *Χριστός *was abbreviated to its initial *Xρ* in old manuscripts. This digraph became the Christogram ☧, symbolizing Jesus on various Christian materials and imagery. The shorter shorthand of *X* for *Christ* also appears in *Xmas*, i.e., *Christmas*, which has been in use since the 1500s. That letter X also resemble a cross, so central to the Christian belief system, further underscores its association with Jesus Christ.

**The Kiss X**

For Christians, Christ—and his symbol X—evoke faith and fidelity, and apparently medieval Christians, few of whom were literate, used X to sign documents as a token of their veracity.

This custom would also appear related to the modern use of X as a signature or in checkboxes, though we should never underestimate that this practice could just be because X is a distinctive and easy-to-form shape to make. Consider how we have X marks spots, dating to at least the early 1800s, which would seem to originate simply from, well, X marking spots.

Important documents were also sometimes signed with X and sealed with a kiss, and eventually the X-as-kiss emerged in letters and, now, text messages. The *Oxford English Dictionary* (*OED*) cites a letter by naturalist Gilbert White in 1763: “I am with many a xxxxxxx and many a Pater noster and Ave Maria, Gil. White.” Others think White’s X’s signified *blessings*, with an 1894 letter from Winston Churchill to his mother marking the earliest known use of the kiss X: “Please excuse bad writing as I am in an awful hurry. (Many kisses.) xxx WSC.”

**The Algebraic X**

X has been standing in for unknown mathematical quantities (variables) since the 17th century, a practice adopted from the French. The *OED* first cites it in English polymath (pun intended) Jonas Moore’s 1660 mathematical treatise, *Arithmetic*.

Why X? Contrary to a popular 2012 TED Talk, the *OED* offers:

The introduction of *x*, *y*, *z* as symbols of unknown quantities is due to Descartes (*Géométrie*, 1637), who, in order to provide symbols of unknowns corresponding to the symbols *a*, *b*, *c* of knowns, took the last letter of the alphabet, *z*, for the first unknown and proceeded backwards to *y* and *x* for the second and third respectively. There is no evidence in support of the hypothesis that *x* is derived ultimately from the mediæval transliteration *xei* of *šay’* ‘thing’, used by the Arabs to denote the unknown quantity, or from the compendium for Latin *res* ‘thing’ or *radix* ‘root’ (resembling a loosely-written *x*), used by mediæval mathematicians.

**X-tensions **

The algebraic X is also responsible for the X* *in *X-ray*, a translation of the German *X-strahlen *(literally “X-beams”). German scientist Wilhelm Röntgen discovered them in 1895 and, taking a page from mathematics, so designated because he didn’t fully understand them.

The “unknown” X also produces that “indefinable quality” of the *X-factor*, which dates back all the way to the 1930s.

X’s sense of mystery probably led to X’s wide use in naming, from the *X-Files* to *SpaceX*, with X here conjuring up a sense of possibility, of wonder. Its associations with words like *extreme* and *extra* further lend it to so much branding (e.g., *Xbox*, *Yukon XL*, *UberX*). Names like *Gas-X* and *Clean-X* play with X as *ex*, or “former,” with X perhaps additionally calling up advanced technology, thanks to the letter’s “unknown” (read futuristic) resonances.

The unknown is also uncertain, which produces the X in Generation X, first used in the 1950s for a “generation of young people about whose future there is uncertainty.” Later uses of Generation X play upon valences of X as anxious or edgy for this supposedly disaffected post-Boomer population.

X is also a prominent sound in *sex. *Perhaps advertisers subliminally take advantage of this connection, but the XXX for pornographic content appears to come from the use of X for motion picture ratings for adult films. (I suspect the choice of X here prudishly alludes to the letter’s connotations with “wrong.”)

XXX, finally, was also used for the potency of various beers in the 19th century, likely leading to the trope of XXX for strong booze in cartoons.

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Due to some upcoming wedding celebrations (I’m honored to be marrying my sister- and soon-to-be brother-in-law this weekend), Mashed Radish will taking Friday off.