Explanation: Bertram introduced the ratios d15,F∕d85,B≈6 and d15,F∕d15,B≈9 as empirical filter designs taking into consideration the grain size distribution. The first criterion was given to ensure that the soil particles do not pass through the voids of the filter (retention or stability criterion) and the second to avoid clogging of the filter, i.e. the filter voids should not be filled with too many fine particles (permeability criterion). Terzaghi & Peck specified more precisely; ● d15,F∕d85,B≤4 ● d15,F∕d15,B≥4