sestet vs sestina

sestet

noun
  • The last six lines of a sonnet, forming two stanzas of three lines each. 

  • A piece of music composed for six voices or six instruments; a sextet or sestuor. 

sestina

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

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

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