Longwall mining is used most efficiently in uniform coal seams (medium height 40 to 60 inches). As in the room–and-pillar method, longwall mining starts with sets of entries cut into the panel areas. The difference in the technique lies in the distance between these sets of entries and the method used to extract intervening coal. Longwall blocks range from 300 to 600 feet wide and are sometimes a mile long. The longwall machine laterally shears or plows coal from the entire face. Then the longwall machine will transport the fallen coal by an advancing conveyor to a secondary haulage conveyor. This reverses direction at the end of a cut and supports the roof around the face by a self-advancing system of hydraulic jacks. Over 80% of the entire coal face can be removed with this method. The roof is allowed to cave-in behind the advancing work areas, and the roof is occasionally blasted to ensure a controlled cave-in rate and to reduce overburden pressure on the coal being mined. The shortwall method of mining coal is best described as a process like longwall mining with two exceptions. One being the blocks of panels are smaller, usually ranging from 100 to 150 feet wide and 300 feet long. The second one being the coal is cut with a continuous miner and is loaded into shuttle cars.