corvee vs serfdom

corvee

noun
  • Unpaid labor required by a feudal lord. 

  • Labor, especially for roads or dams, in lieu of taxes. 

serfdom

noun
  • The feudal system that includes serfs. 

  • The state of being a serf. 

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