This is a cap that screws onto the 1 gallon carboys. The barbed fitting can be used to to connect a blow off tube (3/8 in.) into a glass jar instead of using an airlock. It is intended to be used with something like the link below. It has a very good description on why you'd want a blow off tube vs an airlock. Check it out! http://www.thingiverse.com/thing:42617 . Also consider joining the Home Brew group on Thingiverse: http://www.thingiverse.com/groups/brewing/about