These parts allow you to convert a 3" paper tube to a poster/document tube. The tube was the exact same length as the paper because it was the tube the paper was originally spooled on so this gives you an extra inch or so in length so that you don't crumple the edges and has a screw on cap for easy access. For me the fit of the caps was really tight so I didn't need any glue. The straps tighten down with an M3 machine screw and nut.