Protective Closure feature ensures the adhesive will not come in contact with the contents when re-using the bag. Made from Biaxially Oriented Polypropylene (BOPP), a superior material that does not easily rip, tear, wrinkle, or fog like other plastics. Bag sizes indicate the opening side as the first dimension. 1/16 size tolerance. 6 Mil wall thickness. Archive safe. See recommend size chart for popular sizes.