Harry and Meghan were named the Duke and Duchess of Sussex on their wedding day, and it is traditional for the eldest son of a duke to eventually inherit his father’s title. Down the line, Archie could be given the secondary Sussex title, before inheriting the dukedom.
Traditionally, royal titles are decided by the Queen. After his birth in 2013, Prince George was officially named: His Royal Highness Prince George of Cambridge. Siblings Charlotte and Louis were named: Her Royal Highness Princess Charlotte of Cambridge and His Royal Highness Prince Louis of Cambridge.