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There's an unofficial rule that says two heirs to the royal throne should never be on the same plane together.

The purpose of this is to protect the royal lineage should anything happen.

Considering Prince William is second in line to the throne, followed by Prince George and Princess Charlotte, none of them should actually fly together.

The Queen has the final say if the rule can be broken and she's been pretty lenient thus far.

You know how much we all love watching the royal family — Prince William, the former Kate Middleton and little George and Charlotte — get off planes together when they go on royal tours to countries outside the U.K.?

It turns out, they're not really supposed to do that, and the reason is pretty bleak, TBH.

There's an unofficial rule that says two heirs to the royal throne should never be on the same plane together so they can protect the royal lineage should anything happen to the plane. According to their family tree, Prince William is the second heir, right after his father Prince Charles. Meanwhile, George is the third heir and Charlotte is the fourth. That means that, in theory, all three of them should always be in separate planes, even if they're going to the same place.

Of course, with tots as young as George and Charlotte, that's not exactly practical. And the rule is an unofficial one, which means there's some wiggle room. The first exception for Prince William's family was granted by the queen back in 2013 soon after George was born when the family traveled to Australia and New Zealand. The queen has the final say in when the rule can be broken, and she's been lax about it for Prince William's family — they've since traveled together to Canada, Poland, Germany and other countries.

Even though the reason for the rule is pretty bleak, we have to commend the prince for saying, "Screw it," and keeping his family together. After all, air travel is still a fairly safe way to travel, and with such busy schedules of royal engagements, squeezing in family time wherever possible (even if it's en route to a royal engagement) is a big deal for this happy family.