rectangular triangle or in form of a saw tooth and define a slanted surface 40 in fig5 the projections 20 are of triangular cross section defining a slanted surface 40 and a further slanted surface 42 of sharper inclination persons skilled in the art will understand that fig2 5 show only examples of possible configurations of the projections 20 and other configurations may certainly be conceivable in the embodiments according to fig4 and 5 the screen elements 12 have asymmetric cross section allowing a versatile arrangement of the screen elements 12 for example in fig4 the left and central screen elements are mounted to the cross bar 14 with the slanted surface 40 ascending toward the left while the slanted surface 40 of the right screen element 12 ascends toward the right in fig5 the installation of the screen elements 12 is such that the slanted surface 40 of the left and right screen elements 12 ascends toward the right while the slanted surface 40 of the central screen element 12 ascends toward the left thus when providing the screen elements with asymmetric configuration as shown in the non limiting examples of fig4 and 5 the effective screen area can be selectively designed by differently positioning neighboring screen elements 12 i e through turning the screen elements 12 by 180 u00b0 about the longitudinal axis of the screen element referring now to fig6 there is shown a fragmentary schematic illustration of two neighboring screen elements 12 showing in detail the area of the screening slot 16 between neighboring screen elements 12 for sorting and classifying a fluid flow e g a fiber suspension incoming and flowing in direction of arrow f each.