An Uno fitter is threaded so that it screws onto the bottom end of a socket. Bridge arm lamps are the most common type of lamp that take a shade with an Uno fitter. Bridge arms are usually floor lamps, but can also be table lamps. They have a top arm which extends off to one side of the central column. Bridge arm lamps take a shade with an Uno fitter that is designed to push the light downward. All sizes are top diameter by the bottom diameter and then the height. Fantastic shantung materials.