Styles editor Stella Bugbee wrote a lovely essay on why we — Musk included — are all so obsessed with the letter X. “For marketing purposes in the 1990s, X had a certain cool.

Indeed, Mr. Musk seems to have something of a fixation with the letter X. He named his rocket company SpaceX, a Tesla the Model X, and one of his own children X Æ A-12, or X for short.

X’s ambiguity is what makes the letter so compelling. And, Pelkey noted, that’s part of what makes it a good fit for Musk’s troublemaking, boundary-pushing style: “The X mark is for ...