Later this month—on July 22, to be exact—Prince George will turn 11 years old. Though it’s not an age one would think of when one thinks of a milestone birthday, it is significant, though only to those enmeshed in royal protocol: when George turns 12 in one year’s time, he will have to stop flying with his father, Prince William, to help ensure the future of the monarchy, per The Mirror. 

Heretofore William—who is the first in line to the throne—has been able to travel freely with George and his other two children, Princess Charlotte and Prince Louis, whom he shares with his wife, the Princess of Wales. But, as The Mirror writes, “There is an important royal custom in place to secure the future of the monarchy by restricting who can travel together on the same plane,” the outlet writes. William had to adhere to this rule, as did his father King Charles before him—once an heir to the throne turns 12, he or she is not allowed to ride with any other heirs to the throne.

Speaking on “A Right Royal Podcast,” Charles’ former pilot, Graham Laurie, opened up about this piece of royal protocol and how it affected a young William when he turned 12 in 1994: “Interestingly, we flew all four—the Prince [Charles], the Princess [Diana], Prince William, and Prince Harry, up until Prince William was 12 years old,” he said. “After that, he had to have a separate aircraft and we could only fly all four together when they were young with the written permission of Her Majesty [Queen Elizabeth]. When William became 12, he would fly normally in a 125 from Northolt, and we would fly the 146 out with the other three on.”

While this piece of royal protocol can be broken with the monarch’s permission, “it is discouraged since, in the event of a crash, the future of the monarchy would be immediately unstable,” The Mirror reports.

With past as prologue, that means that this rule will affect George in 2025, and will no longer be allowed to fly with his father. “However, it remains to be seen whether the young prince will continue to travel with his mother and siblings while his father uses a different plane, or if the second-in-line to the throne [George] will be the one to fly solo,” The Mirror writes.

Though, thankfully, plane crashes rarely happen in modern times, the royal family has sadly been impacted by air disasters—Prince Philip’s sister, Princess Cecile, died in a plane crash in 1937; Her late Majesty’s uncle Prince George, Duke of Kent died in 1942 in the same way, and her cousin, Prince William of Gloucester, died while competing in an air show in 1972.

And, as The Mirror reports, it’s not the only travel rule that applies to the royal family and its royal protocol: the monarch always travels with a supply of blood and a doctor with him at all times. You can take that factoid to your next dinner party.