The shapes of particles produced by comminution in the Szego Mill, a planetary ringroller mill, in different coal grinding operations show that particle shape is dependent on the type of operation, and help explain the breakage mechanisms. Dry grinding produces particles with a low aspect ratio due to interparticle attrition; significant briquetting of fine particles may occur. In slurry grinding flaking particles are produced which are selected for breakage at rates lower than those for granular particles. similar observations have been made with other materials. Coal-oil-water grinding is accompanied by the formation of coal-oil agglomerates. their breakage produces uniform particles with a low aspect ratio due to interparticle attrition within the agglomerates.