While it’s not uncommon for an airline to require a certain dress code for passengers, United Airlines has found itself in the social media crosshairs for refusing to allow young girls to board a flight because they were wearing leggings.

Another traveler waiting to board at Denver International Airport started Tweeting on Sunday when she noticed that three young women — two teenagers and one girl around 10 — were not allowed to board because they were wearing stretchy leggings. This was surprising to her, as yoga type pants are a pretty ubiquitous sight these days.

While the younger girl was allowed to board her flight after pulling a dress on over her leggings, the two others were turned away, she said in later Tweets.

The airline responded to her concerns on Twitter and said that the airline has “a right to refuse transport for passengers who passengers who are not properly clothed” via its Contract of Carriage.

“This is left to the discretion of the agents,” United Tweeted.

While United did not define what “properly clothed” means, specifically, the airline clarified that the two passengers who had been barred were traveling on a companion pass. As such, their leggings were in violation for the company’s dress code policy for “pass travelers,” which allows United employees and their dependents to travel for free on standby.

A United spokesman confirmed the events to The Boston Globe, noting that because pass travelers are “representing” the company, they’re not allowed to wear things like spandex leggings, tattered or ripped jeans, midriff shirts, flip-flops or any article of clothing that shows their undergarments.

“It’s not that we want our standby travelers to come in wearing a suit and tie or that sort of thing,” he said. “We want people to be comfortable when they travel as long as its neat and in good taste for that environment.”

The two teen girls were aware of the company policy and “made an adjustment” to their outfits in Denver, he said, and waited for the next flight to Minneapolis. He didn’t know whether or not they were successfully allowed to board that flight.

“Our regular passengers are not going to be denied boarding because they are wearing leggings or yoga pants,” the spokesman told The Washington Post. “But when flying as a pass traveler, we require this pass travelers to follow rules, and that is one of those rules.

The backlash on Twitter was immediate, and included criticism from celebrities like Patricia Arquette and Chrissy Teigen. Many people pointed out that legging and yoga pants are a common sight, including on airplanes where travelers are trying to stay comfortable in tight spaces.

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