backroom vs smoke-filled room

backroom

noun
  • A room near the rear of a premises, especially one that is only accessible to a privileged few and can be used as an inconspicuous meeting place. 

adj
  • Characterized by secrecy or anonymity. 

  • Of, relating to, or happening in a backroom. 

smoke-filled room

noun
  • A place where powerful people meet to decide a matter in secret, often of a political nature. 

  • Used other than figuratively or idiomatically: see smoke, filled, room. 

How often have the words backroom and smoke-filled room occurred in a corpus of books? (source: Google Ngram Viewer )