preface vs prolog

preface

noun
  • The beginning or introductory portion that comes before the main text of a document or book. 

  • An introduction, or series of preliminary remarks. 

  • A title or epithet. 

  • The prelude or introduction to the canon of the Mass. 

verb
  • To give a preface to. 

  • To introduce or make a comment before (the main point). 

prolog

noun
  • A speech or section used as an introduction, especially to a play or novel. 

  • A component of a computer program that prepares the computer to execute a routine. 

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