Fits PET Carboys. A breathable Silicone stopper that acts as both a stopper and an airlock. Unique design allows CO2 pressure to escape but keeps Oxygen and airborne contamination out. Quickly became our stopper of choice for wine. Used by numerous commercial wineries. Also has great application for beer ferments: Will create a vacuum if fermenting wort cools (as opposed to pulling water in from your traditional airlock.) Air tight seal is great for extended fermented lager when CO2 production is still present but very slight.