free verse vs tenson

free verse

noun
  • A poetic form divided into lines of no particular length or meter, without a rhyme scheme. 

tenson

noun
  • A form of lyric poem used by troubadours in Provençal in which two voices present different sides of an argument, in alternate stanzas, lines, or groups of lines with identical structure. 

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