Envelopes with Double Side Seams Some envelopes have vertical seams at both ends of the envelope, rather than diagonal seams. This style of envelope may be more likely to wrinkle. When using envelopes with double side seams, be sure the seam extends all the way to the corner of the envelope as illustrated below. Acceptable Unacceptable Figure 1-3 Envelopes with Double-Side-Seams Envelopes with Adhesive Strips or Flaps Envelopes with a peel-off adhesive strip or with more than one flap that folds over to seal must use adhesives compatible with the heat (400° F (205° C)) and pressure in the printer. The extra flaps and strips might cause wrinkling, creasing, or jams. Paper Specifications 1-21