trefoil vs triumvirate

trefoil

noun
  • Any of several plants of the pea family, having compound, trifoliate leaves; especially one of the genus Trifolium. 

  • A symbol having the shape of such leaves, especially when used as an architectural ornament. 

triumvirate

noun
  • An official group of three people, especially a ruling council of three men and particularly (historical) two such councils in Roman history. 

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