heptad vs trio

heptad

noun
  • Synonym of septet: a group of seven things. 

  • A sequence of seven bases. 

trio

noun
  • A group of three people or things. 

  • A piece of music written for three musicians. 

  • A group of three musicians. 

  • Any cocktail made with a spirit, a liqueur, and a creamy ingredient. 

  • A passage in the middle of a minuet, frequently in a different key. 

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