An air classifier divides material flow into two parts. One of them consists of fine material, the other one of coarser parts. Centrifugal, gravitational and flow forces impact on mass elements which are of di"erent sizes and use natural laws to bring about classification. The “tromp curve“ illustrates how e!ciently the materials are classified: The lower the deviation from the vertical, the more precise the sharpness of classification. The lower the deviation from the horizontal line, the greater the classification effect.