crown prince vs successor

crown prince

noun
  • 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. 

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

successor

noun
  • A person who inherits a title or office. 

  • The next heir in order or succession. 

  • A person or thing that immediately follows another in holding an office or title. 

  • The integer, ordinal number or cardinal number immediately following another. 

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