Fab Four
Prince William and Kate Middleton have to follow an unwritten royal travel rule that Prince Harry and Meghan Markle can break. Pictured: Prince William, Middleton, Markle and Prince Harry attend a Commonwealth Day Service at Westminster Abbey in central London, on March 12, 2018. Getty Images/Paul Grover

Kate Middleton and Prince William have to stick with an unwritten royal protocol that Prince Harry and Meghan Markle can break when traveling.

According to Kara Godfrey, a digital travel reporter for Daily Express, the royals have an unwritten protocol that prohibits the heirs to fly on the same plane in case of an accident of emergency. This aims to prevent the line of succession from being affected in case of a crash.

The Duke and Duchess of Cambridge are discouraged to travel abroad with their children 4-year-old Prince George, 3-year-old Princess Charlotte and newborn Prince Louis. However, the Duke and Duchess of Sussex may do so when they have their own kids as Prince Harry is already sixth-in-line to the throne.

Prince Harry and Markle's children would be much further down the succession compared to Prince George. Prince William and Middleton's eldest child is third in line to the throne and is behind his dad and grandfather Prince Charles. So, he should be protected.

The Duke and Duchess of Cambridge have broken the rule when they traveled to Canada and enjoyed a tour in Australia and New Zealand with Prince George. According to the report, Prince William asked the Queen to allow them to travel with him and his request was granted. Since the said travel rule was not official "it is something that the Queen has the final say on."

In related news, Prince William just turned 36 years old but spent his birthday away from home. On his special day, the Duke of Cambridge attended the unveiling of a world-class rehab facility for Armed Forces personnel who suffered major trauma or injury. During the ceremony, Prince William read "One for the Team" by Debbie Lawson, the winner of the "Poem to Remember" competition.

According to Middleton's husband, his role at the Defense and National Rehabilitation Center (DNRC) gives him "street cred" with his son, Prince George.

"I have been the Patron of the DNRC for 4 years and I have enjoyed every single moment of it," Prince William said. "Along the way, I have demolished a building — George was at an age at the time that he loved seeing the digger in action, so it did my street cred as a father a world of good."