Grand duchy
A Grand Duke (or Grand Duchess) is a person who rules a Grand Duchy.  But in the English language "Grand Duke" can also mean a type of prince who does not rule a country, but is related to the monarch.  In the Russian language Velikiy Knjaz are "Grand Princes", relatives of the Tsar, other princes were noblemen who ranked above a duke.  English says Grand Duke  instead of Grand Prince.