couplet vs song

couplet

noun
  • A pair of lines, typically with rhyming end words. 

  • A pair of one-way streets which carry opposing directions of traffic through gridded urban areas. 

  • A pair of two mutually exclusive choices in a dichotomous key. 

song

noun
  • A musical composition with lyrics for voice or voices, performed by singing. 

  • A low price, especially one under the expected value; chiefly in for a song. 

  • A melodious sound made by a bird, insect, whale or other animal. 

  • An object of derision; a laughing stock. 

  • Poetical composition; poetry; verse. 

  • The distinctive sound that a male bird utters to attract a mate or to protect his territory; contrasts with call; also, similar vocalisations made by female birds. 

  • Any musical composition. 

  • The act or art of singing. 

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