Slip Uno adapters are made to fit directly over a plastic lamp socket found on lamps usually purchased at many popular retail stores. This adapter attaches directly to the harp, allowing use of nearly any replacement lamp shade. This is meant to be an alternative to lamp shades with a built-in fitter. You can now easily attach a harp to a lamp without one. The opening on this adapter is 1 7/16 Inches. A plastic washer (ring) is needed to hold the adapter tightly in place. Please note, there are two fitter sizes (1 and 7/16 inch diameter and a larger 1 and 9/16 inch diameter). Please refer to the website for the larger fitter.