This Carboy Cap fits 3, 5, and 6 gallon carboys (does not fit 6.5 gallon carboys). It fits tightly around the carboy mouth, and the two openings are convenient for attaching or inserting tubes, airlocks, siphons, or racking canes. Caps for both openings are included. More than meets the eye! Of course the Carboy Cap is a cap, and can be used as such. But the beauty of this item is its flexibility and usefulness in combination with other equipment.