If you are inventing your own world, you can call anyone a duke, princess, or whatever you like, as long as you’re consistent within that world.
However, if you are writing a historical novel, or even a semi-historical one, it’s much smarter to follow the correct conventions for titles of nobility.
Back at the turn of the twentieth century in Russia, anyone marrying into or born into a royal family was automatically titled prince or princess. That’s why there were hundreds of Russian princes and princesses hiding out all over Europe when the Russian revolution took place.
British royalty, however, is very different.
You may have noticed that the Queen’s husband is not called the king. He is Prince Philip. He was already a prince before they married, having been born into the Greek royal family. Similarly Prince Charles’ wife, Camilla, is not a princess. She’s the Duchess of Cornwall, having been given one of Prince Charles’ secondary titles. Even Charles’ former wife was not actually Princess Diana, although she was popularly known by that name. Her real name was Diana, Princess of Wales. A tiny difference, but one very important to the British aristocracy.
Also, please never have the beautiful, impoverished, non-noble heroine sitting beside the charming Duke or Marquis for dinner. The seating at dinner followed absolutely unbreakable rules and was strictly by precedence. Hostesses spent hours checking volumes such as “Burke’s Peerage” to ascertain whether Lord Snob’s title was older than Lord Whosit’s. The older the title, the higher on the social scale, so therefore the higher up the table they sat.
Helen is available to line edit and/ or content edit fiction and non-fiction. Rates on application.