Claim envelopes are designed for use with Insurance Form CMS-1500 and similar forms. Window positioning on envelopes allows address on Insurance Form CMS-1500 to show through. No folding is needed. Simply insert form and seal. Redi-Seal closure requires no moisture. Just raise the lower flap and press to seal. Interior is tinted for privacy. Window size is 1" x 4-1/2", positioned 4" from left and 10" from the bottom. 28 lb.