AN rings have corrugations that face inward. They are open in the free state, so that when installed inside a bore, the ring conforms to the bore and is self-retaining. AN rings are designed to fit outside a nominal circumference.

BN rings feature corrugations that face outward. They are designed to have a free state diameter smaller than the shaft diameter over which they will be installed. BN rings are cut to fit inside a nominal circumference.