Particle size, when measured in thin sections, is stated in terms of either the "projected diameter" or one of the so-called statistical diameters. Martin's (1923) statistical diameter is the mean length of a line intercepted by the profile boundary which approximately bisects the area of the profile. With slight modification, Martin's diameter can be used to estimate sphericity distribution along with size distribution; instead of one line, two lines at right angles, long-diameter and short-diameter, are measured for each grain, and cumulative curves are drawn showing each type of distribution. From the scatter diagram of the plot a line of best fit is drawn. Four ideal cases are discussed.