Keepons are made of silicone and they're sold in pairs.
Two identical pieces, one for each of your identical eyeglass temples.
Looking closely at a piece of Keepons, you'll notice that it's basically a hole with a fin on the side.
We call the hole, a Threading Hole.
The fin, we refer to as a Hook.
You use Keepons by passing your temple tip through the threading hole and stopping when the Keepons is situated at your temple bend.
Then when you put on your glasses, the Keepons from its position at the temple bend, hooks over your ears.
Conceptually, the Keepons sits in the middle, one side connected to your glasses, the other side connected to your ears, effectively anchoring your glasses to your ears.
Preventing eyeglass slipping.
However, the way Keepons work is also the cause of why a fitting is needed before buying.
Recall from earlier that to use Keepons you need to insert your temple tip through the threading hole and then position the Keepons at your temple bend where it belongs.
This invites a problem because most eyeglasses have very wide temple tips and very narrow temple bends.
Which means the Keepons threading hole has to stretch really wide to pass the temple tip and then unstretch very tightly to grip the temple bend firmly.
At the end of the day however, a silicone hole can only stretch so big.
We've tried making the material stretchier but overdoing it weakens the material.
Incidentally, this is why we don't actively sell Keepons using size an indicator of which Keepon to buy.
Size is usually associated with the overall dimensions of an item like pizza for example. But with Keepons, what's important is which threading hole when fully-stretched and unstretched is compatible with your eyeglass temple tip diameter and temple bend diameter.
Furthermore, different styles of Keepons have different threading hole diameters with different stretch properties.
Which is why we created our Keepons Fitting Guide.