In the song ‘16 Tons’ there is a verse that goes, 'I loaded 16 tons on number 9 coal." What’s number 9 coal?
Well, the lyrics are actually, IIRC,
That having been said, “number 9 coal” simply refers to coal strip-mined from a seam in Hopkins County, Kentucky, which was called “Number 9”. This sort of numerical designation is not uncommon in Appalachian coals, and the Pittsburg #6 seam is still mined today.