Postal Standards - The Graphic Design: Reference and Specification - page 127

Sealing Self-Mailers (United States)
Folded self-mailers need to be sealed in way that allows them to be easily fed through
the USPS's automated equipment. Consult the diagrams that follow to determine where
wafer seals and tabs should be placed.
Double Postcard Size
Folded Self-Mailer
Multiple-Sheet Mailer