This is a bit of a technical post, provoked by reading a certain novel.
In England, pre-Tudors, there was only ever one Prince. The Prince of Wales, when one existed. The Black Prince; Henry V before his promotion; Henry VI's son; Richard Duke of York (briefly); the future Edward V; finally, Richard III's son. (The Tudors started the habit of applying Prince or Princess to all a sovereign's legitimate children. These were, however, courtesy titles, not peerages.)
Similarly, there was only one Princess, the Princess of Wales. A title held by Joan of Kent, Cecily, Duchess of York, and later, briefly, by Anne Neville. An exceedingly rare distinction. (I suspect that after her husband's death Duchess Cecily preferred to be 'The King's Mother' or even 'Queen by Right'.)
I believe the confusion arises because other persons of high rank, down as far as marquesses, were sometimes referred to as princes or princesses. You will find this, for example on certain tombs.
Well, so they were, at least technically. But they were not referred to as Prince or Princess or addressed as such. It's a subtle difference, I know, but an important one,
To give you a simple example, Elizabeth of York, Duchess of Suffolk was a princess, because she was the sister of the King and the wife of a duke. But she was not Princess Elizabeth and would have been amazed to be addressed as such. But it would be fine to address her in writing as 'the high and mighty princess, Elizabeth, Duchess of Suffolk'. A pedantic point, but an essential one.
