Airlines in the U.S. have been told not to recognize “X” gender markers on passports, prompting confusion and concern among trans and non-binary travellers.

As of this month, a new rule is officially in effect, meaning the U.S.’s Advance Passenger Information System, which airlines use to submit passenger information to the government for international flights, no longer accepts “X” gender markers and mandates that passengers are marked with either an “M” or “F.”

According to U.S. Customs and Border Protection, travellers won’t be denied flying because of the gender marker, but “the carrier or the traveler should select either ‘M’ or ‘F.’”

The new rule was introduced back in July, but with a 90-day grace period for airlines to adjust. That period is now over.

U.S. passports with an X remain valid travel documents—the U.S. district court of Massachusetts issued an order in June ensuring that they would remain valid after the Trump administration attempted to ban them under the president’s gender executive order. As for other countries, Canada has officially issued a warning to travellers that they could face problems entering the U.S. if they have an “X” gender marker on their passport.

But what happens if you do roll up to the airport for an international flight in or out of the U.S. with your X passport in tow? We break down what you need to know.