emir vs grand prince

emir

noun
  • A prince, commander or other leader or ruler in an Islamic nation. 

  • A descendant of the prophet Muhammad. 

grand prince

noun
  • An unusual ruler's title, sometimes incorrectly rendered as grand duke, mainly used in Lithuania and some parts of Russia (Muscovy would ultimately unite most Russian principalities and rise to imperial rank) and the Holy Roman Empire. 

How often have the words emir and grand prince occurred in a corpus of books? (source: Google Ngram Viewer )