On April 2011, after more than six years of dating, Prince William and Kate Middleton got married at the Westminster Abbey in London.
Like many of the royal couples who preceded them, the pair exchanged traditional Anglican wedding vows from the Book of Common Prayer instead of writing their own.
It was nonetheless an emotional moment as millions of people watched William and Kate pledge to be there for each other "for better or for worse," and "in sickness and in health" until the day they die.
However, what many royal fans didn't realize is that the couple had already made promises to each other before their big day, and we now finally know what Prince William said to his wife.
Understanding that marrying into the royal family comes with a unique set of challenges, sacrifices and responsibilities, William reassured Kate that he will do his best to make her transition from girl-next-door to Duchess of Cambridge as easy as possible.
Most importantly, he promised her that she would get to keep at least one important part of her life before she married into the Windsors - her family.
“When William asked Kate to marry him, he made it very clear that she wouldn’t have to leave her family behind once she married into the family,” Vanity Fair writer Katie Nicholl reveals in a documentary titled Kate Middleton: Working Class to Windsor. "He promised her that they’d always stay a part of the family and they have done."
Kate has always been close with her parents, Michael and Carole, as well as her siblings Pippa and James, so it would've been a deal-breaker if they were excluded from her new life.
This promise has put Kate in a unique position because she's one of the only members of the royal family whose parents and siblings are often present at major royal events.
"Whether it’s joining the Queen at Ascot or being on an important barge within the Queen’s fleet at the Diamond Jubilee celebrations, they’ve been there," added Nicholl.
Over the years, the couple have also been spending some holidays with the Middletons, including Christmas. Even though Kate usually joins the Queen at Sandringham for the annual festivities, every few years, she along with William and their children spend time with her parents in Berkshire, England.
While it's unclear if William's brother, Prince Harry, made a similar promise to his new wife Meghan Markle, all signs seems to point to yes.
In an unprecedented move, Queen Elizabeth II reportedly asked Meghan's mother, Doria Ragland, to join them for this year's Christmas celebrations.
Her Majesty has never invited the family of an in-law to spend time at her country home, not even Kate's parents. However, they have previously attended Christmas day church services with the Queen and the rest of the family.
Seeing that this is her first Christmas as a royal, it's possible that the Queen wanted Meghan, whose family live in the United States, to have someone close to celebrate the special day with her.