Western Sulphite® Open Side Booklet with Vertical Window Open Side Booklets have their opening on the long dimension - offering inserting ease and convenience. These envelopes are manufactured with white wove SFI Certified paper, two side seams under construction, a vertical poly window and a traditional remoistening seal.White Sulphite (SFI CERTIFIED) 92% Brightness Vellum Finish Two Side Seams Under Vertical Poly Window Remoistening Seal Flap DownWindow (4 x 1 3/8, 2 1/8L - 5/8B)