heptarchy vs undecimvirate

heptarchy

noun
  • A government of seven people. 

  • The realm so ruled. 

  • A group of seven states, especially (historical) those in Anglo-Saxon Britain. 

undecimvirate

noun
  • A group of eleven people, especially (politics) a council of eleven men sharing office or power and particularly (historical) the groups of eleven magistrates in ancient Athens. 

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