This stopper has two holes and we use it for thermowells. It features one 3/8" hole and one that is 1/4". A racking tube or stainless tube can be inserted into the 1/4" hole with the aid of some petrol gel, forming an airtight seal, and the 3/8" hole fits an airlock. Even after you wedge all these fittings into it, it still fits a plastic carboy! Use this in any configuration you can think of.