feudalism vs serfhood

feudalism

noun
  • A social system based on personal ownership of resources and personal fealty between a suzerain (lord) and a vassal (subject). Defining characteristics are direct ownership of resources, personal loyalty, and a hierarchical social structure reinforced by religion. 

serfhood

noun
  • The role or status of a serf. 

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