letters vs liberal arts

letters

noun
  • The liberal arts, humanities, learning (broad accumulated cultural knowledge). 

  • Literature (school subject). 

liberal arts

noun
  • The academic course of instruction intended to provide general knowledge and usually comprising the arts, humanities, natural sciences, and social sciences, as opposed to professional or technical subjects. 

  • The trivium and the quadrivium. 

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