legal tender vs paper money

legal tender

noun
  • Any form of currency that by law cannot be refused as payment to extinguish a debt equal to the amount offered. 

paper money

noun
  • Cash in the form of banknotes. 

  • A type of joss paper resembling banknotes burnt to venerate the deceased, ancestor spirits, deities, etc. 

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