I don't want to nit-pick, but the nature of Wales's principality is rather different from that of Monaco. The "Prince of Wales" is simply one of the titles of the eldest son of the UK monarch - no eldest son, no PoW (though presumably it would still be referred to as a principality). In Monaco, on other hand, Prince Rainier is the Head of State; and moreover there is provision for female succession, so there could be a Princess of Monaco in her own right, whereas I'm pretty sure that any Princess of Wales, like Diana, would have to be married to the Prince. Incidentally, isn't Lichtenstein a Duchy (ie with a Duke as HoS), rather than a Principality?