duo vs quartet

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. 

quartet

noun
  • A musical composition in four parts, each performed by a single voice or instrument. 

  • A group of four musicians who perform classical music together. 

  • A group of four singers, usually males, who sings together in four-part harmony. 

  • A group of four people or things, particularly 

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