crown prince vs emir

crown prince

noun
  • A person designated and raised to become the next king. 

  • A person expected to inherit or take over an enterprise or undertaking once its current manager retires or dies. 

  • A person designated and raised to become the next emperor. 

emir

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

  • A descendant of the prophet Muhammad. 

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