No, it’s not a macaroni necklace made by Princess Charlotte, or even a pair of uggs for the flight home, but rather an entirely black outfit only to be worn in the very unfortunate case that a family member may die in their absence.
And it’s not a custom exclusive to just Wills and Kate.
In fact, reports reveal the entirety of the British Royal Family must be prepared.
When Princess Diana’s father Lord Spencer sadly passed away while she and Prince Charles were on a skiing trip through the Alps in 1992, the couple were ready with an all-black ensemble for the journey home.
Perhaps the ruling came after Queen Elizabeth II’s father George VI’s death in 1952. When she and Prince Philip were participating in a Kenyan safari, photographers were unable to capture the pair until mourning outfits were brought in for them to wear.
Fans will recall this as a true moment harrowingly told in season one of The Crown.
It’s not the only fascinating royal rule in place - there’s a myriad of intriguing protocol in place that the British family must follow at all times.
Until recent years, direct heirs used to be forbidden to travel together. Before the aviation industry evolved, direct heirs were never put in the same plane together in case it went down for fear of losing two potential rulers in one hit.
While quite a morbid strategy, these days the rule has been relaxed and you’ll often see Prince William and Prince George jetting about in the same aircraft.
And you’ll never catch a royal accepting refreshments during a walk-about tour, no matter how peckish they may be.
It’s not because he doesn’t have a sweet tooth, it’s simply because it’s too much of a security risk to accept food from strangers. So don't be offended if your treat gets tossed, the decision to simply not eat it is a big step up from the way the royal family used to handle the threat - employing “food tasters” to test their meals for poison.
WATCH the video for more of Duchess Catherine's most fashion-forward looks to date!