Since Queen Elizabeth has been ruling for almost 60 years, it's easy to forget that the former system favored a male monarch. The change overturns rules dating back 300 years, which held that a first-born daughter could only become queen if she had no younger brothers. That means if Prince William and Kate Middleton's first-born is a daughter, she will automatically be the presumed heir to the throne, no matter if she has younger brothers.
The prime minister reiterated that point, "put simply, if the Duke and Duchess of Cambridge were to have a little girl, that girl would one day be our queen." And while the idea of a monarch might be pretty retro itself, Cameron said these changes will make it more in line with the times, "The idea that a younger son should become monarch instead of an elder daughter simply because he is a man, or that a future monarch can marry someone of any faith except a Catholic — this way of thinking is at odds with the modern countries that we have become."