Fits 3, 5, 6, and 6.5 gallon glass carboys and acts as both a stopper and an airlock. Its four vents and pliable seal allow CO2 to escape but prevents entry of oxygen and airborne contaiminants. No more suckback of liquid from the airlock as wort cools; great for extended fermentations where CO2 production is slight. Diameter is 1 1/2" at the top, 1 1/16" at the bottom.