room-and-pil|lar

room-and-pil|lar «ROOM uhn PIHL uhr, RUM-», adjective.

of or designating a system of mining coal in which a series of rooms is cut into the coal bed, in each of which the miners leave columns of coal standing to help support the roof until they mine out a particular area. Most underground coal mines in the United States are room-and-pillar mines.