Room-and-pillar mining is the most common type of system for mining coal. In this system, the mine roof is supported mainly by coal pillars left at regular intervals. Rooms are places where the coal is mined; pillars are areas of coal left between the rooms.
Room-and-pillar mining is done either by 1) conventional mining, which involves a series of operations that require cutting the working face of the coalbed so that it breaks easily when blasted with explosives or high-pressure air, and then loading the broken coal or 2) continuous mining, in which a continuous mining machine extracts and removes coal from the working face in one operation. When a section of a mine has been fully developed, additional coal is extracted by mining the supportive pillars until the roof caves in; the procedure is called room-and-pillar retreat mining. In some cases, corrosive materials, such as acids, are used for the final removal of the pillars. In these cases, the technique is referred to as Corrosive or Subtractive Mining.
This method of coal extraction is known as bord-and-pillar in the United Kingdom