cathedra vs regius professor

cathedra

noun
  • The official chair of some position or office, as of a professor. 

  • The rank of bishop. 

  • The chair or throne of a bishop. 

regius professor

noun
  • A professor who holds a position created by or filled by a royal patron. 

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