This rank is reserved for someone who is heiress to the kingdom throne and therefore known as Crown Princess, or a ruler of a principality. The king and queen themselves are the prince and princess of any Crown Principality that may be in existence at a given moment.
The princess is accorded very high rank as a member of a kingdom's nobility. The Crown Princess ranks immediately after the king and queen, together with the Crown Prince, and is addressed as Your Royal Highness. The princess of a principality ranks immediately after the Crown Prince and princess, together with their prince, and is addressed as Your Serene Highness.