Before Prince William's firstborn child was born, the Queen made a significant amendment which would change the course of history forever.
This decision means that Prince Louis, Prince William and Kate Middleton's third child, is the first male not to be ahead of his female sister in the Royal line of succession.
Prince George, the first child of the Duke and Duchess of Cambridge, is currently third in line to the throne – followed by his sister Princess Charlotte, who is fourth, after which comes George in fifth place.
This had not been the case if the Queen hadn't passed the Succession to the Crown Act in 2013.
In the Act, the Queen scrapped what is called the male-preference primogeniture, which favours male children as heirs to the throne even if they have an older sister, for Prince William's children.
It meant that William's first born child would be a direct heir to the throne – even if the child was female.
That means that Princess Charlotte comes ahead of Prince Louis in the line of succession.
Before the change Charlotte would have dropped down a place in the line of succession after Louis was born, because she is a girl.
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This is why Princess Anne is further away from being Queen than her younger brothers Prince Michael and Prince Andrew.
The change now means age comes before sex, a change that was hailed at the time as fairer for female Royals, and in line with modern opinion of the roles of men and women in society.
All three of William and Kate's children are still ahead of Prince Harry and his son Archie, who are sixth and seventh in line respectively.
When Prince Harry and Meghan Markle have their daughter this summer, she will take the eight spot, bumping down Prince Andrew from his current position.
The Queen also made a legal change to allow Louis and Charlotte to have HRH titles, which Archie doesn't have.
Previously this would only have been granted George as the Cambridge's firstborn child.
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This was a rule created by King George V, which restricted the use of HRH titles to only the monarch, the monarch's children, grandchildren and the monarch's eldest-born's first grandson.
Under those rules, Charlotte and Louis would be Lord and Lady Mountbatten-Windsor. But the Queen changed the rule in 2012.
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