The proportion of a coal seam that is removed from a mine. The remainder may represent coal in pillars or coal that is too thin or inferior to mine or is lost in mining. Shallow coal mines working under townships, reservoirs, etc., may extract only about 50% of the entire seam, the remainder being left as pillars to protect the surface. Under favorable conditions, longwall conveyor mining may extract from 80% to 95% of the entire seam. With pillar methods of working, the extraction ranges from 50% to 90%, depending on local conditions.