back-alley vs backstairs

back-alley

adj
  • Occurring away from publicity, or in mean surroundings; backstreet. 

backstairs

adj
  • Secret or furtive. 

  • Scandalous. 

noun
  • An indirect or furtive means of access or intercourse. 

  • A staircase at the rear of a building or one normally only used by servants and tradesmen. 

How often have the words back-alley and backstairs occurred in a corpus of books? (source: Google Ngram Viewer )