Lady Louise Windsor may be four years older than her brother James, Earl of Wessex, but due to an antiquated and traditional law, Louise is actually further down in the line of succession to the British throne than her younger sibling.
The 20-year-old daughter of the Duke and Duchess of Edinburgh, who is studying at the University of St Andrews, is currently 16th in line to the throne, while her brother James, 16, is above her in 15th place.
At the time of Louise's birth, the Succession to the Crown Act had not come into effect, meaning that male primogeniture, under which a younger son can displace an elder daughter in the line of succession, was still in place.
Changes to the Succession to the Crown Act
The Succession to the Crown Act was only introduced in 2013, just three months before the Princess of Wales gave birth to her son, Prince George.
Rumours swirled that Princess Kate may have been expecting a girl during her first pregnancy, because the law was changed just months before she gave birth and therefore it would have protected her first-born's place in the line of succession, had George been in fact a girl.
It changed the laws of succession to the British throne in accordance with the 2011 Perth Agreement, effectively meaning that the eldest child, regardless of gender, precedes any siblings.
Princess Charlotte makes history
Royal babies born after 28 October 2011 were affected. Consequently, Princess Charlotte, who was born in May 2015, retained her place in the line of succession and was not overtaken by her younger brother Prince Louis, who was born in April 2018.
Similarly, Zara and Mike Tindall's daughters, Mia, ten, and Lena, five, also benefited from the Act and kept their places above their baby brother, three-year-old Lucas.
Lady Louise, however, falls behind her younger brother James in the order of succession because she was born in 2003, two decades before the law was altered. Other female royals who are in the same position are Princess Anne, who is 17th in line to the British throne and whose younger brothers Prince Andrew and Prince Edward are ahead of her in eighth and 14th place, respectively.
Louise and James's titles
When Louise and her brother James's parents married in 1999, they agreed with the late Queen that their offspring would be styled as children of an Earl rather than a Prince.
They were therefore known as Lady Louise Mountbatten Windsor and James, Viscount Severn although James's title changed to James, Earl of Wessex when his father Edward became the new Duke of Edinburgh on his 59th birthday in March 2023 and following the death of both of his parents, the late Queen and Prince Philip.
The pair were given HRH titles at birth which they could choose to use when they turned 18, but so far, Louise has chosen not to do so.
Will the Edinburgh children work in the future?
Their mother Sophie has previously spoken about her and her husband's down-to-earth attitude towards raising children, telling The Sunday Times: "We try to bring them up with the understanding they are very likely to have to work for a living.
"Hence we made the decision not to use HRH titles. They have them and can decide to use them from 18, but I think it's highly unlikely."