It may matter what air travelers wear on the plane.

That’s not just the case for the legging-wearing teenage girls denied boarding on a United Airlines flight this spring. The two girls were flying on United’s pass program, which lets friends or relatives of airline employees fly free or at discounted rates. The catch is a dress code.

But even for those paying regular fares, there is a dress policy, of sorts. United, Delta Air Lines and American Airlines all list bare feet as grounds for removal from flights in their official contract of carriage documents, for example. United’s policy also covers passengers “not properly clothed,” and American’s warns that the airline may refuse to transport passengers “clothed in a manner that would cause discomfort or offense to other passengers.”

Those extra words, according to Brian Sumers, a reporter at the travel industry website Skift, mean that anyone who shows up in a T-shirt scrawled with profanity or a lewd message may be barred from a plane. Those in bikini tops may also have a problem, he said. But beyond that, he doubts that modern airline employees pay much attention to what passengers wear.

“You know, people would have told you two decades ago that it was a good idea to dress nicely on a plane, because you never know if the airline might take care of you, like maybe bump you up to business class or first class because you look nice,” he said. “But that doesn’t really happen now. Airlines know who their best customers are, and they’re the ones they’ll bring up if there’s a seat available.”