pentarchy vs undecimvirate

pentarchy

noun
  • A governing body consisting of five persons. 

  • Government by five persons. 

  • A federation of five nations, each under its own government or ruler. 

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 pentarchy and undecimvirate occurred in a corpus of books? (source: Google Ngram Viewer )