Design updated: Made of polyester. The opening of the bags is sewn with loop strips which help you quickly put these vent bags on or off the vent rain cover. Convenient to use: No other items such as string, clamp, magnets, wire or screw be needed if you use our vent cover bags. The hook and loop fasteners would do the job. Size: This vent bag are designed to fit 4 inch diameter dryer vent pipe, 4 inch diameter vent elbow and 4 inch dryer vent hood. Application: the problem of birds' nests or dryer lint dust. The lint bags can not only catch the lint but also allow proper venting.