front line vs lines

front line

noun
  • A front, or a boundary between opposing positions. 

  • A low level. 

  • attack, collectively the attackers or forwards. 

  • A site of a conflict, effort, or controversial matter of any kind. 

  • The site of interaction with outsiders, such as customers. 

lines

noun
  • Dispositions made to cover extended positions, and presenting a front in but one direction to an enemy. 

  • The reins with which a horse is guided by its driver. 

  • A school punishment in which a student must repeatedly write out a line of text related to the offence (e.g. "I must be quiet in class") a specified number of times; the lines of text so written out. 

  • Words spoken by the actors. 

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