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Meaning of room-and-pillar

room-and-pil•lar

Pronunciation: (rm'un-pil'ur, room'-), [key]
— adj. Mining.
  1. noting a means of extracting coal or other minerals from underground deposits by first cutting out rooms, then robbing the pillars between them; pillar-and-breast. Cf.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.