my Lord vs my Lord Duke

my Lord

noun
  • Used in direct address to bishops, particularly Anglican ones. 

  • Used in direct address to peers of the realm, usually now excluding dukes and the Lords Spiritual. 

  • Used in direct address to the Lord Mayors of certain important cities, including London and Canterbury. 

  • Used in direct address to certain judges, particularly High Court Judges and above. 

my Lord Duke

noun
  • Used in direct address to dukes, now chiefly in correspondence. 

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