If you have type 688
bearings, you will need this adapter to fit the bearings in an inline skate wheel.
Because inline skate wheels are made for type 608
These adapters are sold per single piece. So for a pair of inline skates with a standard set-up of 4 wheels per inline skate, you need 16 adapters.
(One of the pictures shows the use of the adapter with a bearing. Of course the bearing is not part of the article).
An example of a bearing for which you need the Powerslide Wicked 688 to 608 adapter
is Wicked Jesa Swiss Mini 688 bearing