Seams and seals are what give an envelope its unique shape and size. Envelope seams are typically permanently sealed for durability. Seams variations include:

Center Seams
Are most commonly used on coin and catalog envelopes.

Diagonal Seams
Used on commercials and pointed flap baronials.

Side Seams
Are formed by short side flaps or a wide bottom flap, providing a large area for printing on the back of the envelope, as in reverse face and collection envelopes. Side seams may be folded under or over the bottom flap, or have a single side seam.

Weld Seams
These seams eliminate the die cutting operation and can be produced to permit insertion and trimming in a magazine. The envelope's pocket size is determined by the distance between the internal gum lines.