With the release of Walter Isaacson's salacious new Elon Musk biography, renewed focus is being paid to various aspects of Musk's personal life and upbringing. Amongst the areas of interest are where Elon Musk's obsession with the letter X stems from. In typical Elon fashion, there isn't one particular reason or inspiration, but rather a smattering of significant personal associations with the 24th letter of the alphabet.

In addition to the biography, a recent New Yorker profile of Musk by renowned deep dive journalist Ronan Farrow, and another by Jill Lepore, delve into his ‘X' fascination. All of these pieces point out that Elon Musk's first company was X.com, an early online bank, which merged with Peter Thiel's Confinity, and eventually became PayPal. This was the first instance of Musk publicly utilizing the letter for branding, but his interest in ‘X' has far earlier historical roots.

It goes all the way back to 1637 when French mathematician wrote a treatise on geometry, introducing the now-popularly used math variables of x, y and z. In an amusing bit of early technological difficulties twist, Descartes' printer was having difficult setting the type for Ys and Zs, but not Xs — since X is not a commonly used letter in French vocabulary — so Descartes mainly stuck with using X's to represent the unknown entity. X as the variable symbol quickly also became synonymous with mystery and the unknown, at least in mathematical circles (with Musk certainly frequents in).

A couple hundred years later, in 1895, the German physicist Wilhelm Röntgen discovered a new kind of radiation, but didn't know what it was yet, so he decided to brand it with a fittingly mysterious name — the X-ray.

A few years later, in the 1930s, Elon Musk's grandfather became a leader of the Technocracy movement, as detailed in Jill Lepore's five-part BBC Radio 4/Pushkin podcast series called “Elon Musk: The Evening Rocket.” As Lepore explains, the Technocracy movement “believed that only engineers and scientists could save the world from a looming catastrophe.”

Further, “in the future that Technocrats including Musk’s grandfather were planning for, ‘There will be no place for Politics or Politicians, Finance or Financiers, Rackets or Racketeers.' There would also be no place for personal names.”

This led one Technocrat to change his name to 1x1809x56, which undoubtedly influenced Elon Musk's decision to name one of his own 11 children, X Æ A-12— or X, for short.

The unusual name choice may also be related to the interests of the baby's mom (and Musk's on-again, off-again partner), the artist/musician Grimes. Like Musk, Grimes is a big fan of science fiction and may have read a famous piece of feminist sci-fi published in Ms. magazine in 1972. In the piece, a baby is named X in order to not reveal the child's gender, as part of a secret scientific experiment.

A few years later, in a case of life imitating art, the fictional piece gave way to an actual scientific experiment building on the story's theme, and the results were subsequently published in a journal article called Baby X.

Lepore's New Yorker piece posits that Grimes is “the kind of person who pretty plausibly would have read the story of X in college, either the feminist fable, or the actual experiment, or both.”

Also, when the baby was born, Musk said it was a boy, though Grimes told fans, “I don’t want to gender them in case that’s not how they feel in their life.”

This bit of news is also ironic in light of the fact that another of Musk's children, his teenage daughter Vivian, has come out as trans and recently changed her last name because, according to the legal filing, “I no longer live with or wish to be related to my biological father in any way, shape or form.”

In any case, X as a prominent reference in the world of sci-fi entertainment — not just in the Baby X example, but also major franchises like X-Men, The X-Files, and the video game system Xbox — may also figure into Musk's obsession.

X is also associated with the scandalous and erotica (as in X-rated or XXX). Someone who has 11 kids with three different women and developed the four main models of Tesla cars to spell out the word S-3-X-Y certainly must have an affinity for this meaning of the letter as well. (No, that's not a typo, by the way — the Model 3 would have been an E if another car company didn't already have claim to the letter.)

This all brings us to the present, and Musk's current obsession with rebranding Twitter as X. Elon Musk confesses to desperately trying to make his mammoth social media purchase “the everything app.” If he indeed succeeds in someday bridging all his varied interests under one ‘X' umbrella, it will be the result of a lifelong, and generation spanning, fascination with the letter.