Kings, queens, princes, princesses – it all sounds like a fairytale, doesn’t it? And that’s probably the biggest reason that people in America are so obsessed with the royal family, even if they don’t realize it. Classic Disney films have inspired all of us to dream about the seemingly magical world of royalty.

But here in the real world, there are certain rules that pertain to titles. Centuries of tradition dictate exactly who gets called what, and becoming a princess isn’t as easy – or as black and white – as it seems. So why was Diana a princess, but Kate Middleton a duchess? The answer is a little trickier than you think.

How to become a prince or princess

The easiest way to become a prince or princess is to be born to a prince. That’s why there’s no mistaking that Prince William’s children – Prince George, Princess Charlotte, and Prince Louis – are all bona fide royalty with spots in the line of succession. Their titles are clear.

Here’s one thing to keep in mind though: only the sons and daughters of princes inherit the title of prince or princess. That’s why Princess Charlotte’s future children won’t bear that official title, even though she is a princess. Remember, these customs were established long before women’s equality was a thing.

Marrying into the royal family

The other way to become a princess is by marrying a prince. So why don’t we ever refer to Kate Middleton as, “Princess Kate?” It’s all just a matter of custom. The Duchess of Cambridge is technically a princess, though it’s improper to address her as such. Same goes for Prince Harry’s wife, Megan Markle, who is officially called the Duchess of Sussex.

Officially Kate Middleton is known as Her Royal Highness, Duchess of Cambridge. Since she wasn’t born into the royal family, the title “Princess” would not come before her name. She is a princess, though – Prince Louis’ birth certificate lists her official occupation as, “Princess of the United Kingdom.”

A word on Princess Diana

So if you have to be born into royalty to be called, “Princess So and So,” why did everyone call Diana Spencer Princess Diana? It’s an easy answer: because the people loved her.

Even Diana admitted that referring to her as Princess Di was technically incorrect. Her proper title was Diana, Princess of Wales or Her Royal Highness the Princess of Wales since she married the Prince of Wales. It’s unlikely that Princess Diana minded being called by an incorrect title, but it probably didn’t do much to help the strained relationship between the Queen and her son’s wife.

What about other titles?

A royal child’s parents have the option of calling them prince or princess or choosing to do away with that title altogether. Prince Andrew decided to keep the title for his daughters, Princess Beatrice and Princess Eugenie, but Prince Edward opted to forgo the title for his daughter, Louise Windsor. She received the title of Lady Louise instead.

Royal titles of king, queen, prince, and princess rank highest in the hierarchy. Just below that are dukes and duchesses, followed by marquess and marchioness, earl and countess, viscount and viscountess, and baron and baroness. All nobles are called lords and ladies except for dukes and duchesses, who are referred to as, “Your Grace.”

The Queen has the final say

Many of these titles are steeped in tradition, which is why even in our modern age the women of the royal family aren’t afforded the same exact title granting abilities as the men. Still, Queen Elizabeth has the power to change any of these rules whenever she wants and bestow titles on anyone she chooses.

When Princess Anne’s children were born, the Queen offered to give them titles of Prince and Princess even though that was technically breaking protocol since Princess Anne is female. However, Princess Anne and Captain Phillips politely declined her offer.

It’s a long way off, but the reigning king when Princess Charlotte has children – whether it’s her father, Prince William, or her brother, Prince George – could offer her children the same title. Then it will be up to her to decide if she wants her little ones to be called prince or princess, just like she was.