folk song vs saga

folk song

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

  • folk music 

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

saga

noun
  • An Old Norse (Icelandic) prose narrative, especially one dealing with family or social histories and legends. 

  • Something with the qualities of such a saga; an epic, a long story. 

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