The U.S. Supreme Court seemed to struggle Monday with rules for trademarking generic names that are combined with the .com phrase.

Many justices seemed to agree with an attorney for Booking.com that a 132-year-old Supreme Court precedent probably doesn't control the case. But they also sounded wary of granting an online monopoly over entire categories of goods and services, and that it could lead to a flood of litigation over similar sounding dot.com names.

"If you succeed in trademarking Booking.com," Chief Justice John Roberts told Williams & Connolly partner Lisa Blatt, representing Booking.com, then competitors like ebooking.com and Eurobooking.com "will be impeded from using that term, which is an accurate description of the services and goods that they provide."

That concern is just a hypothetical laid out in the U.S. Trademark and Patent Office's brief, Blatt argued. "They don't give you a single example of harm despite the ubiquity of generic-word.com marks."

"You are seeking a degree of monopoly power that nobody could have had prior to the internet age," Justice Samuel Alito told Blatt. "Because of the internet, you have Booking.com, which gives you an advantage over other companies that are in that business. And now you want to get even more advantage by getting trademark protection for that."

Blatt argued that the PTO has registered many generic dot.coms, such as Weather.com, and that they've peacefully coexisted with similar named companies, such as Accuweather. "You wouldn't want to write an opinion destroying the billions of dollars of goodwill that's been built up" in generic marks tied to dot.com, or tied to "company" or "store," without any showing of anticompetitive harm, Blatt said.

Blatt is bearing the weight not only of Booking.com, which says it's invested $5 billion in advertising and marketing its name, but also a virtual army of businesses who've either registered or want to register a generic name combined with dot.com. The PTO argues that such names are per se generic. Salesforce.com is headlining one amicus curiae brief, while Cars.com, Dictionary.com and Wine.com are among 15 others that have banded together in another.

Without federal registration, a valid trademark can still be enforced against would-be infringers. But federal registration confers additional legal rights and benefits. It provides constructive notice of the mark, it can become incontestable once registered for five years, and the trademark holder can stop the importation of counterfeits.

Especially important to Booking.com is being able to block companies that engage in spoofing and typo-squatting of domain names, which Blatt said require a registered trademark to fight.

A longstanding rule of trademarks is that generic names cannot be registered. The PTO argues that adding .com isn't enough to make a generic mark descriptive and potentially eligible for registration. The agency is invoking an 1888 Supreme Court ruling, Goodyear's India Rubber Glove Mfg. v. Goodyear Rubber, that adopted a similar rule for the addition of "Company" or "Inc." to a name. More recently, the U.S. Court of Appeals for the Federal Circuit has upheld the PTO's refusal to register Hotels.com and Mattress.com, and the U.S. Court of Appeals for the Ninth Circuit has found AOL's Advertising.com generic.

The agency is getting amicus support from the Electronic Frontier Foundation and a group of intellectual property scholars led by Harvard's Rebecca Tushnet, whose brief the justices brought up repeatedly Monday. The scholars caution against giving generic mark holders too big of a stick to ward off competitors and suggest that .coms can use unfair competition law to solve the problems they've identified.

Booking.com won a split decision at the U.S. Court of Appeals for the Fourth Circuit. It argues a lot has changed in the 130 years since Goodyear's was decided. The Lanham Act, the nation's 1946 trademark statute, "repudiated" Goodyear's "root to branch," Blatt said Monday. Under the statute, the question is whether the primary significance of a mark is that consumers identify it with a single brand. And Booking.com submitted a consumer survey that showed 74% identified it as a brand, compared to only 30% who incorrectly identified Washingmachine.com as a brand.

The justices sounded uncomfortable with the bright-line rule set by Goodyear's. "Let's put Goodyear's aside for the moment," Justice Neil Gorsuch told DOJ attorney Erica Ross, who argued for the PTO.

"Suppose you lose on your argument for a categorical rule," Justice Elena Kagan said.

Ross argued that "what Booking.com wants is something it couldn't get in the brick-and-mortar world." The dot.com owner already has a first-move advantage by being the first to register the domain name, she said. Adding trademark protection presents "a very real risk of monopolization."

Breyer suggested it's the PTO that's trying to carve an exception to the ordinary rule. "You can have a trademark that's an address. You can have a trademark that's a phone number. So why can't you have a trademark that's a dot.com?"

But Justice Clarence Thomas—more talkative than usual because of the orderly turns each justice took in asking questions under the telephonic format—said that Blatt seemed to be relying heavily herself on the primary significance test.

And Justice Elena Kagan said there "seems a disconnect between the primary significance test and these kinds of names." Because dot.com names are by definition unique, the results of survey tests are likely to be biased, she said.

Justice Brett Kavanaugh said that under Blatt's rule, dot.coms that sells goods or services could be registered every time. "Maybe that's OK, but if that's wrong, when wouldn't it?" he asked.

"I think it's wrong, but I hear at least six of you concerned about it, so I can try to help on this," Blatt said. The PTO can look past survey evidence to many other sources "if you're really worried about a survey bias."

Justice Neil Gorsuch was the one member of the court who seemed firmly in Blatt's camp. "The relative weakness of the mark together with the fair use doctrine is a response to the government's monopoly concerns. Is that a fair summary?" he asked.

"Much better than I said," said Blatt.

Pillsbury Winthrop Shaw Pittman partner William Atkins, who like many listened the arguments live on C-SPAN, said it was a pleasant surprise to hear Thomas ask not one, but numerous questions.

"It was great to hear Justice Breyer follow up Justice Thomas' question about why one can register a 1-800 number as a trademark and yet not a dot-com," Atkins said. The government's response was "not as compelling," he said.