oblique mark vs underscore

oblique mark

noun
  • Synonym of slash ⟨/⟩, sometimes (generally proscribed) inclusive of similar marks such as the backslash ⟨\⟩. 

underscore

noun
  • An underline; a line drawn or printed beneath text; the character _. 

  • A piece of background music. 

verb
  • To emphasize or draw attention to. 

  • To underline; to mark a line beneath text. 

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