oblique mark vs perpendicular

oblique mark

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

perpendicular

noun
  • A device such as a plumb line that is used in making or marking a perpendicular line. 

  • A line or plane that is perpendicular to another. 

adj
  • at or forming a right angle (to something). 

  • Exactly upright; extending in a straight line toward the centre of the earth, etc. 

  • Independent of or irrelevant to each other; orthogonal. 

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