empress vs grand dukedom

empress

noun
  • The female monarch (ruler) of an empire. 

  • The third trump or major arcana card of most tarot decks. 

  • A female chimpanzee. 

  • A deciduous tree, Paulownia tomentosa 

  • The wife or widow of an emperor or equated ruler. 

grand dukedom

noun
  • The title of grand duke or grand duchess, with its pertaining privileges and obligations; contrast grand duchy. 

How often have the words empress and grand dukedom occurred in a corpus of books? (source: Google Ngram Viewer )