Prince Harry and Meghan Markle keep interpreting royal traditions with a modern twist. While they did a postbaby photocall in a real-life castle, they also debuted their baby's name on Instagram just like many nonroyal parents do. That name — Archie Harrison Mountbatten-Windsor — basically confirms reports that baby Archie will not use a royal title. Mountbatten-Windsor is the official last name used for members of the queen's family who do not have the "royal highness" styling. While he could have been called "Lord Archie" by virtue of being Prince Charles's grandchild, it seems Harry and Meghan's son will be known by the more "simple" title of Master Archie. That will change in the future, however, and he'll technically become Prince Archie!
Master Archie will become Prince Archie when Charles becomes king.
Per the rules of peerage, which dictate British royal titles, Archie will become a prince when his grandfather Prince Charles becomes king. The current rules, issued by King George V in 1917, state that the title of prince or princess goes to the sons and daughters of the sovereign, grandchildren born to the sovereign's sons, and the eldest son of the Prince of Wales's eldest son (who would be Prince George today). This is why Prince Andrew's children, Princess Beatrice and Princess Eugenie, are princesses, while Princess Anne's children, Peter and Zara Phillips, are not. The queen reportedly offered a "courtesy" title to Zara and Peter, but Princess Anne apparently declined.
The queen did make a change in 2012, decreeing that all children born to the eldest son of the Prince of Wales (aka Prince William) would have the title prince or princess upon birth. Thus, we have Princess Charlotte and Prince Louis. If the queen offered to do the same for Harry's children, it appears he and Meghan have turned her down. Assuming no more changes to the rules are made, Master Archie would become a prince when Charles becomes king — since the 1917 rule states that grandchildren born to the sovereign's sons are princes or princess. Archie could also inherit the Duke of Sussex and other titles from Harry one day. And today, Archie is already entitled to use one of Harry's "lesser" titles, like the Earl of Dumbarton, but it appears they have decided not to use that either.
Of course, Archie could decline to use the prince title when grandpa Charles becomes king. Today, Kate, Meghan, and even Charles's wife, Camilla, use their duchess titles instead of the princess title they have by virtue of being married to a prince. (You may recall Kate's occupation was listed as "princess of the United Kingdom" on Prince George's birth certificate.) Whether or not he uses it, we think His Royal Highness Prince Archie has a good ring to it.