The Duke and Duchess of Cambridge, Prince William and Kate Middleton, don't actually have legal custody of their three children George, Charlotte and Louis - and the same will go for Prince Harry and Meghan Markle for their future kids.
The custody of the children, in fact, belongs to their great grandmother, Queen Elizabeth II.
"The sovereign has legal custody of the minor grandchildren," royal expert Marlene Koenig told news.com.au.
"This goes back to King George I [who ruled in the early 1700s], and the law's never been changed. He did it because he had a very poor relationship with his son, the future King George II, so they had this law passed that meant the King was the guardian of his grandchildren."
The law is more than 300 years old and was passed by a majority of 10 out of 12 judges in 1717, who decided the monarch's "right of supervision extended to his grandchildren and this right of right belongs to His Majesty, King of the Realm, even during their father's lifetime".
According to Koenig, the law was legislated once again in 1772, when King George III was in power, and has never been superseded by new legislation.
So the custody law still stands today, and has affected the way the royals have parented their children in terms of upbringing, travel and education.
"When [Princes Harry and William] were little, Prince Charles asked the Queen if both children could fly on a plane together to Scotland, to which the Queen said yes. Technically, they needed permission for travel. The Queen has the last word on parenting decisions like that," Koenig said.
Princess Diana was also not allowed to fly with her children to Australia shortly before her death because the Queen did not give permission. Prince Charles also had to ask for permission before sending teenage William off on a holiday camp in the US in the late 1990s.
The divorce between the Duke and Duchess of Wales, Prince Charles and Diana, in 1996 was also affected by the odd custody arrangement.
"Custody is not included [in those divorce documents] because they did not legally have custody of their children to begin with," Koenig said.
This also explains why Princess Diana's wishes for her brother and mother to be Harry and William's legal guardians, as stated in her will, were ignored. Diana didn't have final say in her children's upbringing - the Queen did.
According to Koenig, in the event of the Queen passing, the legal custody of any grandchildren in the palace will be passed on to their grandfather Prince Charles, who would then be King. This would go for George, Charlotte, Louis, and any future children of Meghan and Harry.
Luckily the palace doesn't make a big deal out of the law these days says Koenig, and doubts Charles would ever overshadow important or meaningful decisions - "[He is] very respectful of his son's parenting".
"He understands they want to raise their children privately … the only thing Charles might ask for is more pictures," she joked.