Re: Mya Diva?
Its a flexible rubber grommet. It can expand to fit anything from 1.25in to 1.5in in diameter, which accommodates most bottles. The grommet is also tappered, so it can fit inside larger openings. It was designed to fit as many bottles as possible.
The seal is airtight, as the rubber contracts around the mouth. The hose is washable as well (its a MYA style hose).