Taken from the DMM © USPS, January 2006

“Unenveloped letter-size mailpieces prepared for automation rate mailings must be secured (tabbed) to prevent an open edge from jamming high-speed processing equipment. Standards for tabbing are based on basis weight of paper stock used and the location of the folded or bound edge. As an alternative to tabs or wafer seals, the open edge of the length of the mailpiece may be continuously glued or spot glued. Continuous glue or spot glue is permissible with single-sheet self-mailers and cards.”