When Kate Middleton and Prince William said "I do" back in 2011, the headlines couldn't help but laud the great "Princess Kate." As the years have passed though, that nickname didn't really stick, mostly because it's completely inaccurate. Middleton isn't formally known as a princess, despite being a royal married to a prince. But why?

Princess Diana retained her regal moniker despite marrying into (and later divorcing out of) the royal family, so why is Kate so different?

Kate Middleton lead
Credit: Chris Jackson

“While Catherine is absolutely a princess, her correct title is ‘Her Royal Highness the Duchess of Cambridge,’” CNN royal expert Victoria Arbiter explained to Yahoo Style.

“She wasn’t born a blood princess, so she is not a princess in her own right. When she married William, she took on the rank of her husband, a royal prince. However, referring to her as ‘Princess Kate’ is simply incorrect.”

So why did Diana get a pass? Well, she didn't. The public dubbed her "Princess Di" and unlike with Kate, Diana's name stuck. Even so, Diana was not officially "Princess Diana."

Her official title was "Her Royal Highness The Princess Of Wales," but in British royal tradition, that didn't make her a princess. (Confusing, we know.) Only women born into the royal family like Princess Charlotte or Princess Anne can formally add the title ahead of their names. Who knew?