Screening involves two processes, stratification and passage of fine particles through the apertures. The stratification produces a bottom layer of fine material. This paper deals with the influence of distribution and bed thickness on the rates of stratification and passage. The initial proportion of fine particles in the bottom layer is determined by the feed rate and the initial share of fine material. The rates of stratification and passage are determined by two rate parameters. The stratification parameter is determined here by the proportion of fine particles, the size of the particles in relation to the surrounding material, and the thickness of the upper material layer. The passage parameter for a given screen depends on the size of the particles relative to that of the apertures and the thickness of the material layer on the screen. Here, the particles of different sizes are divided into fractions. This makes possible a prediction of the grading curve of the material at any point along the screen. It is also possible to determine the influence of the feed rate and varying proportions of fine material. For a given screen, bed thickness is directly related to the feed rate. The model has been implemented in a calculation program. The calculations are in general accord with Svedala＇s experimental results. (C) 2000