Abstract: Expansible gusset envelope and method and apparatus for fabricating same from a single blank by inwardly, next outwardly, and then inwardly folding bottom gusset sections along triple transverse score lines to bring a first panel hinged thereto into overlying disposition with a second panel. Thereafter, side flaps are glued to the first panel by inwardly, then outwardly, and next inwardly folding side gusset sections hinged to the second panel along triple longitudinal score lines. Triangular corner tabs hinged to the side gusset sections are glued to congruent triangular portions at the margins of the bottom gusset sections by opening break-away flaps at the margins of the first panel between steps (1) and (2) to expose the triangular portions preparatory to tucking in of the corner tabs whereby all gussets are inwardly collapsed with closed corners at completion.