This rank is reserved for someone who is heir to the kingdom throne and therefore known as Crown Prince, 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 prince is accorded very high rank as a member of a kingdom's nobility. The Crown Prince ranks immediately after the king and queen, together with the Crown Princess, and is addressed as Your Royal Highness. The prince of a principality ranks immediately after the Crown Prince and Princess, together with their princess, and is addressed as Your Serene Highness.