a cappella vs sestina

a cappella

noun
  • A vocal performance with no instrumental accompaniment. 

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. 

adj
  • Singing solely or mainly without instrumental accompaniment. 

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

sestina

noun
  • A chord comprising the first six members of the harmonic series. 

  • A highly structured poem consisting of six six-line stanzas followed by a tercet or envoy, for a total of thirty-nine lines. 

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