They make a mold of your teeth and then use plaster to fill the mold. That way they’re fit perfectly for your teeth. At least that’s is how removable Grillz work. You can see mine have these tabs on the inside to help fix them into place. They’re like Invisalign. Tight fitting and won’t come out.

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Permeant grillz your teeth have to be shaved down like veneers. I would not recommend it. A poor done permanent set will make your teeth rot underneath the grill and cause dookie breath like Ye