Coalification involves the genetic and metamorphic history of coal beds. The plant materials that form coal may be, in part, simply incorporated, or they may be present in vitrinized or fusinized form. Materials contributing to coal differ in their response to diagenetic and metamorphic agencies and the three essential processes of coalification are called incorporation, vitrinization, and fusinization. The processes are defined on the basis of important petrographic characteristics. A diagram is given illustrating the extent of change in the plant material, in comparison with diagenetic and metamorphic intensity. Since coal beds generally include various proportions of the diverse materials resulting from the three processes, practical classification of coal is intimately related to the part each process has played.