Among the fossil fuels, the progression from natural gas to petroleum to coal is one of increasing complexity. Even a wet, sour gas has only a small number of possible components. Once the gas has been treated and purified for distribution to consumers, it typically contains >90% of a single compound, methane. Gas contains no inorganic impurities that might leave an ash residue on combustion. Petroleum usually is a homogeneous liquid with a narrow range of elemental composition – about 82–87% carbon, 12–15% hydrogen and the balance nitrogen, sulfur, and oxygen – with atomic H/C ratio of ≈1.5–1.8. On a molecular level, petroleum contains thousands of individual compounds, every one of which could be separated, at least in principle, using common techniques of the organic chemistry laboratory, and identified [A]. Inorganic ash-forming constituents are commonly less than 0.1%. In contrast, coals have an extremely wide range of composition, some 65–95% carbon, 2–6% hydrogen, up to about 30% oxygen, and possibly several percent each of sulfur and nitrogen. The H/C ratio is less than 1. Coals are opaque, heterogeneous solids. Coals cannot be distilled reversibly. Coals are not completely soluble in any solvent, and even the partial solubility in various solvents is an extraction of components rather than a true, reversible dissolution process. Coals have a macromolecular structure that varies from one coal to another and that has never been completely elucidated for any coal. Coals contain a variable, but appreciable, amount of inorganic material, so that burning a particular coal leaves an ash residue that represents anywhere from a few percent to over 25% of the original weight of the coal. Coals also contain some variable amount of water as they are mined from the Earth, from several percent to about 70%.

Despite the complexity of coals and the difficulties encountered in studying them, systems for classifying and describing coals are nevertheless needed. Such systems can provide a conceptual framework for organizing knowledge of coal composition and properties. In a very practical sense such systems provide the descriptions needed for legally binding buying and selling of coals.