Used to provide a positive seal on your brewing vessel. Drilled hole is compatible with standard Fermentation Airlocks. BPA-free and food grade. 

 

#6 size pairs with most 1-gallon growlers (top diameter 1.25", bottom diameter 1.03", length 1").

 

#7 size pairs with most 3 and 5 gallon glass carboys (top diameter 1.43", bottom diameter 1.19", length 1").

 

Make it a Brew Your Own Tool Kit with a 1 Gallon Growler, Siphon, Airlock, Tubing, and Bottle Filler.