triarchy vs undecimvirate

triarchy

noun
  • Government by three persons; a triumvirate. 

  • A country under three rulers. 

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