This stopper has two holes and we use it in the production of our stopper thermowell. It features one 3/8" hole and one 1/4" hole. A racking tube or stainless tube can be inserted into the 1/4" hole with the aid of some lubrifilm, forming an airtight seal, and the 3/8" hole fits an airlock. Even after you wedge all this junk into it, it still fits a carboy neck! Use this in any configuration you can think of, it'll fit: Airlocks, Thermowells, Racking Canes, and anything else you can imagine!