Better Bottle Closures are the next generation in carboy fittings. These are PET fittings made of the same material as the Better Bottles themselves, and designed to fit snugly into the necks of all sizes of Better Bottles. Most closures such as silicone or Plastic stoppers, will allow oxygen to permeate through, slowly degrading your wine! Better Bottle PET Closures are designed to negate oxygen transfer, resulting in the finest wine you can make! ,These are designed to be compatible with various other Better Bottle Fittings. We recommend purchasing a Dry Trap Airlock to compliment this product, allowing you to conduct Oxygen Free Fermentations!