bar line vs underscore

bar line

noun
  • A vertical line in musical notation used to separate two bars or measures. 

  • Synonym of crossline (“type of headline”) 

underscore

noun
  • An underline; a line drawn or printed beneath text; the character _. 

  • A piece of background music. 

verb
  • To emphasize or draw attention to. 

  • To underline; to mark a line beneath text. 

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