Abstract The literature that describes natural fractures (cleats) in coal can be divided into two areas of activity, including an older mining industry viewpoint and a much newer petroleum industry viewpoint. Cleats in coal have been of interest to the coal mining community for more than 100 years. Knowledge of cleat geometry is important in designing coal mines for maximum extraction efficiency and for safety considerations. One of the earliest references on cleats in coal from the mining perspective is by Mammatt (1834, cited in Kendall and Briggs, 1933), who noted the uniform strike of cleats in coal fields of the United Kingdom. Since that time a large number of papers on cleats have been published. Because of the relatively recent interest in commercial production of coal gas resources, many contemporary research efforts have been directed toward the development of a better understanding of factors that influence the production of gas from coal reservoirs. Among the more important of these factors is the cleat system. The importance of cleat development stems from the fact that the principal natural fracture permeability pathways for the flow of gas and water are through the cleat systems. Cleat permeability is often the reservoir characteristic that has the greatest influence on the economic success or failure of coal gas exploration and development programs (Dhir et al., 1991a, b). A better understanding of cleat genesis is therefore useful for reasons besides academic interest, which include the following: (1) One can better assess why certain coal gas plays