in a bag making machine the longitudinal side edges and adjacent portions of an elongate plastic web , which is folded in half medially of its centerline , pass from a set of inlet rolls over and under , respectively , a pair of vertically spaced separating rolls before merging at a set of outlet rolls . the separating arms extend inwardly between the two separated portions of the web only part way toward the fold line , and are spaced apart exactly twice the depth of the gusset that is to be formed . the remaining portions of the web , which are bisected by the folded line , are disposed in a vertical plane transverse to the separating arms , and upon traveling from the arms to the outlet rolls are folded inwardly between the merging portions of the web by the inner , pointed end of a triangularly shaped plow , which engages and progressively urges the fold line inwardly to the desired gusset depth the separating arms are vertically adjustable relative to each other ; and the plow is adjustable transversely of the fold line of the web selectively to adjust the depth of the gusset .