How Prince William and Prince George unexpectedly broke royal protocol.

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It’s safe to say that being a royal comes with a whole lot of royal rules and traditions.

From strict dress codes to even a ban on Monopoly, there’s a lot to take in.

But while a ban on Monopoly sounds ridiculous, some royal rules are to be taken much more seriously than others.

Yesterday, the royal family attended the Queen’s annual pre-Christmas lunch at Buckingham Palace.

The Queen was joined by Prince Harry and Meghan Markle as well as Prince William and Kate Middleton and their children.

But when the Duke and Duchess of Cambridge drove into the event with their kids Prince George and Princess Charlotte sitting in the backseat, they broke a very important royal rule.

You see, heirs to the throne are often not allowed to travel together in case of an accident.

And although the rule originally extended to just air travel, the protocol has since been extended to travel by car following the tragic death of Princess Diana.

As Prince William, Prince George and Princess Charlotte are all heirs to the throne, they're technically not allowed to travel all together.

According to The Sun, the rule first came about when air travel was still a fairly risky mode of transport. so that the royal lineage would not be affected.

The rule meant that neither the King or Queen or their heirs could travel on a plane together.

Now, with air travel being much safer, the rule has been relaxed.

What actually happens at a royal Christmas? Post continues below...

In 2014, the Duke and Duchess of Cambridge even travelled to Australia together with Prince George.

At the time, a royal spokesperson confirmed: "While there is no official rule on this, it is something that the Queen has the final say on."

The rule has also been reportedly relaxed around car travel.

"Nowadays, it would appear that [travelling together] by car doesn't carry the same issue [as by plane], but they will of course have security police protection," royal etiquette expert Alexandra Messervy told InStyle.

"I suppose they believe that things are relatively safe now."

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