A system of fastening inserted corner plates of first and second duct sections together for assembling the duct sections , comprising: the inserted corner plates having holes extending therethrough between opposite surfaces thereof, respectively, positioned to be aligned when the duct sections are positioned for assembly; a screw having a head and a shank extending therefrom having an unthreaded space adjacent to the head and a threaded portion extending from the unthreaded space to a tip of the shank, the threaded portion being configured to threadedly engage the holes of the inserted corner plates , respectively, the unthreaded space of the screw having a length between an endmost thread of the threaded portion and the head about equal to a thickness between the opposite surfaces of the inserted corner plate of the first duct section such that the space will be located in the hole thereof when the screw is fully received therein to threadedly disengage therefrom such that the screw will be rotatable relative thereto while threadedly engaged with the inserted corner plate of the second duct section to bring the duct sections together, characterized by the endmost thread including an extension configured to bear against one of the opposite surfaces when the screw is fully received in the hole of the corner plate of the first duct section in cooperation with the head , to hold the screw in a substantially perpendicular orientation to at least one of the opposite surfaces thereof and prevent re-entry of the corner plate of the first duct section into a groove between the endmost thread and a next adjacent thread to prevent threaded re-engagement with the threaded portion during rotation of the screw . the inserted corner plates having holes extending therethrough between opposite surfaces thereof, respectively, positioned to be aligned when the duct sections are positioned for assembly; a screw having a head and a shank extending therefrom having an unthreaded space adjacent to the head and a threaded portion extending from the unthreaded space to a tip of the shank, the threaded portion being configured to threadedly engage the holes of the inserted corner plates , respectively, the unthreaded space of the screw having a length between an endmost thread of the threaded portion and the head about equal to a thickness between the opposite surfaces of the inserted corner plate of the first duct section such that the space will be located in the hole thereof when the screw is fully received therein to threadedly disengage therefrom such that the screw will be rotatable relative thereto while threadedly engaged with the inserted corner plate of the second duct section to bring the duct sections together, characterized by the endmost thread including an extension configured to bear against one of the opposite surfaces when the screw is fully received in the hole of the corner plate of the first duct section in cooperation with the head , to hold the screw in a substantially perpendicular orientation to at least one of the opposite surfaces thereof and prevent re-entry of the corner plate of the first duct section into a groove between the endmost thread and a next adjacent thread to prevent threaded re-engagement with the threaded portion during rotation of the screw . <EOS>