On some lampshades--most commonly pendant lampshades--you may require a bottom diffuser. The purpose of the bottom diffuser is to "hide" the socket and light bulb of the fixture when viewed from below. A bottom diffuser is often slightly smaller in size than the bottom dimension of your lampshade.
If you order an optional fabric diffuser with your lampshade, the lampshade diffuser will be constructed of lampshade ring with plastic-backed fabric taped to the ring. The diffuser will be slightly smaller (typically 1/4 inch) in size than the bottom dimension of the lampshade, and the diffuser will rest on the bottom ring of your lampshade.
These diffusers do not come with a hole in the center. If you require a hole, please indicate the size of hole you need in the Product Comment box.