middle name vs pen name

middle name

noun
  • A name between the first given name and the family name or surname; a second or subsequent given name. 

  • A trait, activity, or action closely associated with or aptly describing a person. 

  • One's mother's maiden name. 

pen name

noun
  • A fictitious name used by an author in place of their actual name; a writer's pseudonym. 

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