For decades, coal mining in the United States has involved mining away from the shaft bottom or drift mouth in a series of tunnels called entries. The height of the seam of coal usually determines the entry height, which can range from 813 mm (32 in.) to 6.1 m (20 ft) or more. The width is approximately 6.1 m (20 ft), depending on the mining method and conditions of the mine roof. Usually there are multiple parallel entries in a mine with crosscuts driven at intervals to connect the entries together so mining can proceed more efficiently. The block of coal created by the entry and cross cuts is called a pillar.