duo vs septet

duo

noun
  • A song in two parts; a duet. 

  • Two people who work or collaborate together as partners; especially, those who perform music together. 

  • Any cocktail consisting of a spirit and a liqueur. 

  • Any pair of two people. 

septet

noun
  • A composition for a group of seven musicians. 

  • A group of seven musicians. 

  • A set of seven lines. 

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