The supporting layer of the filter is usually made of a stronger screen or perforated sheet. The filtering layer of technical cloth is on the external or internal side of the filter (according to the direction of the flow of the medium). The filtering layer may be equipped with another protective screen layer to prevent damage during manipulation or the back flow of the medium. The edges of the filter may be reinforced. The most common materials are DIN 1.4301 and DIN 1.4401.