An edge stiffening device for the openings of plastic bags, or the edges of sheets or similar materials comprising a band of stable, plastic material which is easily deformable by hand, said band having a longitudinally extending fold-line and being foldable into U-form. The surfaces of the band which form the inside of the U are provided with self-adhesive layers covered by releasable protective strips. The edge stiffener is plastically deformable by hand and can be used to give a plastic bag, sheet or similar material a stable edge having a desired configuration. To improve the stability of the form, the plastic band may be provided with one or more reinforcements, for example, of metal.