Business forms mailer and related manufacturing process

A business forms mailer construction comprising an upper panel (12), at least one intermediate panel (14), and a lower panel (16); the upper panel is adhesively secured to the at least one intermediate panel, and the at least one intermediate panel is adhesively secured to the lower panel by glue lines (34, 36, 66, 68), one of which (68) extends along one side of the intermediate and lower panels. Also provided are a pair of slitter guide marks (72, 74) applied to one of the upper and lower panels, on either side of the glue line, so that by alignment of a slitting mechanism with the inner of the pair of guide marks, the mailer may be opened along one side for the insertion of one or more additional sheets. Alignment with the outer of the guide marks results in removal of a marginal portion of the mailer, but with the latter remaining sealed.