If you’re flying Spirit Airlines, wearing a crop top that reveals your torso or having certain tattoos could now be enough to get you kicked off a flight.

The airline recently updated its policies to explicitly outline unacceptable passenger attire and appearance, going beyond the vague policies held by most other airlines . In its contract of carriage, which is a legal document outlining airline and passenger responsibilities, Spirit says “a guest shall not be permitted to board the aircraft or may be required to leave an aircraft” if the passenger is “inadequately clothed” or “whose clothing or article, including body art, is lewd, obscene or offensive in nature.”

Inadequately clothed is now defined — having breasts, buttocks or other private parts exposed, or wearing sheer apparel. But what exactly is an offensive tattoo is unclear.

Clarifying all of this tends to fall to airline employees, including the flight crew. In recent years, Spirit and other airlines have stopped passengers from boarding their flights for all kinds of attire or lack thereof: leggings, clothes printed with expletive-laded slogans, short shorts and not wearing a bra.

Overall, most airlines leave a lot open to interpretation, said Bobby Laurie, a former flight attendant and a co-host of the travel show “The Jet Set.”

“It’s very subjective,” Mr. Laurie said. “What someone might find offensive or lewd, someone else may not.” Generally, a decision to remove a traveler from a flight would require consensus from multiple airline employees, he added.

Here’s what some major domestic airlines say about passenger dress code in their contracts of carriage.

Spirit Airlines

The airline says it will also bar barefoot passengers from boarding.

Southwest Airlines

Passengers can’t wear clothes that are “lewd, obscene, or patently offensive,” the airline states, with no further detail.

United Airlines

The airline says that it will not transport barefoot passengers, or passengers who aren’t “properly clothed” or whose clothing is “lewd, obscene or offensive.”

Delta Air Lines

Attire that leads to an “unreasonable risk of offensive or annoyance to other passengers” is enough to get a passenger removed, according to Delta’s contract of carriage.

American Airlines

American asks passengers to “dress appropriately.” This means no bare feet or “offensive” clothing.

JetBlue Airways

Travelers over 5 years old can’t be barefoot, the airline says. Again, no “lewd, obscene, or patently offensive” attire.

Alaska Airlines

Alaska Airlines also prohibits passengers from being barefoot and will remove travelers whose attire creates “an unreasonable risk of offense or annoyance to other passengers.”

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