Prince William and Prince Charles each faces the prospect of someday being King of the United Kingdom. As future British Monarchs, reports said that they have to follow strict royal rules in preparation for their roles.

One of these "surprising" rules centers on their traveling routines. The Daily Express said that the Prince of Wales and the Duke of Cambridge are not allowed to travel together.

Despite going to the same venues, royal rules restrict them from getting on board on the same plane or car. In the instance they absolutely must be together for their travels, they must first ask permission from Queen Elizabeth II.

The publication described the reason behind this as "morbid." As reported, Prince Charles and Prince William are prohibited from traveling together to "protect" and preserve the line of succession.

Reports added that if the two Wales Princes are to travel together, there will be risks of disrupting and damaging the royal lineage. So to avoid that from happening, especially if there is an emergency, they are required to travel separately, at all times.

This royal rule does not only apply to Prince Charles and Prince William. The same publication added that this applies to all direct heirs of Queen Elizabeth II.

In the line of succession, Prince George, Princess Charlotte, and Prince Louis come after their father, the Duke of Cambridge. Therefore, they are not technically allowed to travel together as a family.

Cheat Sheet revealed, though, that the Cambridge family has repeatedly broken this protocol. There were several trips when the Duke and Duchess of Cambridge traveled outside the United Kingdom, alongside their three kids, particularly the eldest, Prince George.

Aside from the Cambridge family, the Wales household has also reportedly broken the same royal rule in the past. The same publication said that Princess Diana used to travel and fly with her two sons, Prince William and Prince Harry.

During that time, both of the Wales Princes were direct heirs of Queen Elizabeth II. They were the second and third persons in line to the British Throne. 

When Prince Charles would need to fly abroad with his sons, they reportedly had to ask permission from the British Monarch. But there was only one time when the Queen granted their request, royal expert Marlene Koenig claimed.

These days, though, the royal rule about traveling might have loosened its stance over Prince William and Prince Harry. As it happens, the Duke of Sussex is now the sixth person in line to the British Crown. Based on the direct line of succession, he is no longer a direct heir since his brother now has kids of his own.