boards vs seventh art

boards

noun
  • A stage (as in a theater). 

  • A hardcover binding on a book. 

  • Examinations given for entry to college or to qualify for a profession. 

  • Structure around a rink for ice hockey. 

seventh art

noun
  • Chiefly preceded by the: the art of making motion pictures; filmmaking. 

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