The adapter ring should always be used with the Philips Avent Classic feeding bottle as part of the Classic two-piece anti-colic system.
Allows air into the bottle instead of your baby's tummy
The two piece Philips Avent anti-colic system consists of the Avent Nipple and adapter ring. As your baby feeds, the unique skirt on the Avent Nipple flexes to allow air into the baby bottle instead of your baby’s tummy. Like breastfeeding, your baby controls the milk flow.