Archie and Lilibet become Prince and Princess under the Letters Patent

Meghan Markle and Prince Harry's eldest child Archie, under the Letters Patent, is eligible to be known as His Royal Highness Prince Archie of Sussex, while Lilibet can be styled as Her Royal Highness Princess Lilibet of Sussex.

King George V issued the Letters Patent in 1917, which stipulates who gets a royal title within the Royal Family. 

When Lilibet and Archie were born, they were great-grandchildren of Queen Elizabeth II, meaning they could not be a Prince or Princess. But, now King Charles III has begun his reign and there have been several key title changes for them.

However, it remains to be seen whether the royal children will use their titles going forward or remain Master and Miss.

King George V issued a Letters Patent in 1917 to restrict who could hold a royal title, and aside from the monarch's children and grandchildren through the male line, only the eldest son of the eldest son of the Prince of Wales was eligible for a Prince or Princess title.

The late Queen Elizabeth II reportedly amended this rule to include all of the children of the eldest son of the Prince of Wales. For this reason, all of the Cambridge children hold Prince or Princess titles, whereas previously only Prince George would have been eligible.