Convex - this allows the eye to blink. It has a slight rounded point in the middle and the eye patch is about 1" thick.
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Flat (stuffed with cotton) - this does not allow the eye to blink. It is stuffed with cotton and the eye patch is about 1/2" thick.
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Flat (foam insert) - this does not allow the eye to blink. It has a foam insert and the eye patch is about 1/4" thick.
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Both flat eye patches can be worn under eye glasses. The convex one can be worn under some eye glasses, the type of glasses that are a little further from the eye.
How do I know what elastic length to get?
The elastic length sizes are a world wide average for men, women and children, and does include the eye patch itself too. For a perfect fit, you can measure your head completely around - how you would wear the eye patch (over ears, under ears, etc) and put that length in the comments section when ordering an eye patch.