postscript vs preface

postscript

noun
  • An addition to a story, play, etc. after its completion. 

  • An addendum to a letter, added after the author's signature. 

verb
  • To extend (a letter or another document) with additional remarks. 

preface

noun
  • An introduction, or series of preliminary remarks. 

  • A title or epithet. 

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

  • 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). 

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