The ‘spare to the heir’ is a well-known royal phrase. It means that any child born after their sibling - the heir - is a second to them or seen as a spare part if something should happen.
While there have been many famous spares throughout history including the late Queen's father, King George VI, and her sister, Princess Margaret, the word is now largely tied to Prince Harry's controversial memoir, Spare.
So how does this affect the likes of Princess Charlotte and her younger brother Prince Louis? A royal expert claimed the Prince and Princess of Wales will be striving to “normalise” their lives with the hope that they never feel like working as royals is their only option.
While Princess Charlotte is expected to eventually take on a role which is very similar to that of Princess Anne, one commentator suggested that Prince William and Kate will be careful to ensure that Prince Louis does not "go down the Duke of York path" and become a "royal hanger-on".
Read more on ok.co.uk