approval vs piece of paper

approval

noun
  • Something mailed by a seller to a collector to match their stated interests; the collector can approve of or return the item. 

  • An expression granting permission; an indication of agreement with a proposal; an acknowledgement that a person, thing, or event meets requirements. 

  • An expression of favorable acceptance and encouragement; a compliment that also condones. 

piece of paper

noun
  • A marriage certificate. 

  • A single sheet or scrap of paper. 

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