The stages of physical and chemical changes that proceed from peat through coal are called coalification, and are classified and described by rank. In general, rank categories are defined based on recognizable changes in coal parameters. Slightly different categories and parameters are used in different countries, but in general, proceed in increasing rank from lignite, to sub-bituminous, to bituminous, to anthracite coal. Different ranks of coal cannot always be differentiated based only on their physical appearance. Testing of different parameters is often needed to differentiate coal ranks.

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Different ranks of coal:

Coal can also be defined and classified according to:

• Coal grade (based on its chemistry and market use)
• Coal type/lithotype (based on its physical layering and composition