Throughout the coal mining areas in the USA there are seams of coal which run for miles and miles in various directions. A multitude of mines tap into a single seam by way of shafts or through strip-mining (depending upon the depth of the seam in a particular area). These various seams are numbered. For instance, in Illinois the #5 and #6 seams are the most productive. In western Kentucky the #9 is mined heavily.