On April 23, 2018 at 10:01 a.m, the Duke and Duchess of Cambridge—Prince William and Kate Middleton—welcomed their third child into the world: Prince Louis Arthur Charles of Cambridge. The moment Louis entered the world, joining siblings Prince George—third in line to the British throne—and Princess Charlotte—fourth in line to the British throne—he not only attracted the attention of millions of people across the globe, but also made history.

Prior to 2015, due to male-preference primogeniture laws set in place, Princess Charlotte would have lost her place in succession to the throne to Louis. All throughout British history, males were always in front of their female siblings when it came to succession to the throne, regardless of whether or not they were older. For instance, Queen Elizabeth II has four children: Prince Charles, born in 1948, Princess Anne, born in 1950, Prince Andrew, born in 1960, and Prince Edward, born in 1964. Although Princess Anne is older than two of her brothers, she is currently thirteenth in line to the British throne while Prince Andrew and Prince Edward are seventh and tenth in line respectively. Princess Charlotte would have faced this same predicament had the Perth Agreement not been made by the prime ministers of the 16 Commonwealth realms—the United Kingdom, Canada, Australia, New Zealand, Jamaica, Barbados, the Bahamas, Grenada, Papua New Guinea, the Solomon Islands, Tuvalu, Saint Lucia, Saint Vincent and the Grenadines, Belize, Antigua and Barbuda, and Saint Kitts and Nevis—in October of 2011.

The Perth Agreement, which replaced male-preference primogeniture with absolute primogeniture, essentially eliminated the notion that sex is relevant in determining inheritance to the royal throne, which in turn ended the discriminatory practices against women in royal families. The concept of absolute primogeniture, or inheritance by oldest child without regard to sex, is so new that not a single modern monarch has done this prior to 1980. The first countries to replace male-preference primogeniture with absolute primogeniture were Sweden in 1980, the Netherlands in 1983, Norway in 1990, Belgium in 1991, Demark in 2009, Luxembourg in 2011, and then the United Kingdom in 2015. Supporters of equal rights for women will be glad to know that thanks to this change, it can be ensured that in the future, females born into a royal family will not lose their place in the line of succession to a younger brother.