According to my father, the manager of permitting and land acquisition for a mid-sized coal company, the No. 9 coal seam is the predominant coal seam throughout the Illinois Coal Basin which covers Indiana, Illinois, and Kentucky. The numbering system is complicated as it varies from state to state (for example, Kentucky No. 9 coal is No. 5 coal in Indiana), but in all states, the lower the number, the deeper the coal. For instance, 13 coal is at a higher elevation than the 9. Coal located at different strata has different properties, but the nine doesn't signify it's ash content. In Tennessee Ernie Ford's "Sixteen Tons" when he says, "I load sixteen tons of No. 9 coal," he is indicating that he is working in the most commonly mined strata in the Illinois Coal Basin, probably in an underground mine.