X.com was the domain name associated with the payment service that Musk founded in 1999; it remained with PayPal after the companies merged. By the time PayPal first went public in 2002, Musk had left—but made millions off that IPO. The change in the domain’s ownership was first reported by Elliot Silver, president of Top Notch Domains, a Boston-based domain management company. He noticed on July 5 that x.com’s information had gone private.

It’s unclear how much Musk spent for the domain name—which is one of just three available single-letter .com domains—but Silver estimated it could be as much as an eight-digit figure ($10 million).

Apart from x.com, z.com and q.com, no other single-letter .com domain names are available to be registered and traded, according to Alan Dunn, the managing director of NameCorp, a Florida-based digital naming agency. Instead, they have been reserved since December 1993 by the Internet Assigned Numbers Authority (IANA), a US-based organization that coordinates elements of domain name use and global IP allocation.

In 2014, z.com was sold to a Japanese domain name registration company for $6.8 million. And q.com is associated with Qwest Communications, a Colorado-based telecommunication company, according to domainIQ, which tracks the domain information. There isn’t an available record of q.com being transferred, according to Zhang Ming, the founder of Mimayi, a Chinese startup that registers domain names.

X.com doesn’t load as of July 11. A Tesla spokesperson referred Quartz to Musk’s tweet and didn’t provide further details. PayPal’s spokeswoman Amanda Miller wrote to Quartz that PayPal was “delighted to sell the domain back to its previous owner.”

“It’s extremely hard to estimate the sales price of private domain name sales, especially when there are intangible (and emotional) X factors in a purchase such as this one,” said Dunn in an email to Quartz. “It’s something which drives a lot of curious people crazy.”

The piece has been updated with response from PayPal on July 11.

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