folk song vs madrigal

folk song

noun
  • folk music 

  • A song, in any folk style, originating among ordinary people, usually non-professional musicians, and handed down by oral tradition. 

  • A song in the style of folk music, a modern genre of popular music. 

madrigal

noun
  • A short poem, often pastoral, and suitable to be set to music. 

  • A song for a small number of unaccompanied voices; from 13th century Italy. 

  • A polyphonic song for about six voices, from 16th century Italy. 

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