Differential thermal analysis, using conventional equipment, has been used to differentiate between ranks of coal. Five distinct thermal curve types are recognized, based upon physical, chemical, and structural changes in coals of different ranks. Classification of curve type is based principally upon the number and temperature of endothermic devolatilization thermal peaks.Natural rank boundaries, where curve types change, occur between meta-anthracite and anthracite, semi-anthracite and low-volatile bituminous, medium-volatile bituminous and high-volatile bituminous, and high-volatile bituminous and subbituminous. Gradational boundaries with no change in curve type occur between anthracite and semi-anthracite, and low-volatile and medium-volatile bituminous. The change in thermal curve type agrees favorably with A.S.T.M. rank boundaries.Thermal curves indicate that the coalification process is gradual with increase in rank to anthracite. At this point, a structural "break" separates meta-anthracite from the rest of the sequence.