a cappella vs duet

a cappella

noun
  • A vocal performance with no instrumental accompaniment. 

adj
  • Singing solely or mainly without instrumental accompaniment. 

  • Related to a form of purely vocal music mostly associated with American college performance groups. 

adv
  • In a manner of a choir with no instrumental accompaniment; literally, "in the style of the (Sistine) Chapel (in Rome)", such as a musical Mass done a cappella. 

duet

noun
  • A musical composition in two parts, each performed by a single voice (singer, instrument or univoce ensemble). 

  • A song composed for and/or performed by a duo. 

  • A pair or couple, especially one that is harmonious or elegant. 

verb
  • To perform (sing, play, etc.) as a duet. 

  • (of two people) To say at the same time, to chorus. 

  • To perform a duet. 

  • To communicate (warnings, mating calls, etc.) through song. 

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