corpus vs gravamen

corpus

noun
  • A collection of writings, often on a specific topic, of a specific genre, from a specific demographic or a particular author, etc. 

  • Such a collection in form of an electronic database used for linguistic analyses. 

  • A body, a collection. 

gravamen

noun
  • The essence or most important aspect of a piece of writing, a point of argument, etc.; the gist. 

  • A grievance complained of. 

  • The essence or ground of a complaint. 

  • A document sent by the Lower House of Convocation to the Upper House to inform the latter of certain grievances in the church. 

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