prequel vs sequel

prequel

noun
  • In a series of works, an installment that is set chronologically before its predecessor, especially the original narrative or (perhaps improper usage) any narrative work with at least one sequel. 

sequel

noun
  • A narrative that is written after another narrative set in the same universe, especially a narrative that is chronologically set after its predecessors, or (perhaps improper usage) any narrative that has a preceding narrative of its own. 

  • The remainder of the text; what follows. Used exclusively in the set phrase "in the sequel". 

  • Thirlage. 

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