Sedimentary particles are formed by disruption of rocks by physical forces. This random process results in predictable probabilities of occurrence of various particle shapes which may be classified in terms of combinations of three axial lengths, a, b, and c into discs, spheres, blades and rods.. The shapes with the highest probability of occurrence fall within the limits of the blade class in which a is greater than b is greater than c. Departures froom the expected frequencies of occurrence of the randomly produced shapes indicate nonrandom influences such as the effects on rock fracturing of anisotropic ccrystal structure and slaty cleavage.