On some lamp shades--generally on pendant lamp shades--you may request 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 lamp shade.
If you order an optional diffuser with your shade, the diffuser will be constructed of a ring slightly smaller in size than the bottom dimension of the shade, and small spokes will be added to the bottom ring to hold the diffuser up.
Optional diffusers for shades with a bottom diameter of less than 26" will measure approximately 1" less in diameter than the bottom diameter. Diffusers for shades with a bottom diameter of 26" or more will measure approximately 2" less in diameter than the bottom diameter.