3 and 4 Inch | Lip and Straight Fitters
3 1/4inch fitter has an outside glass diameter of approximately 3 inch
4" fitters have an approximate outside glass diameter of 3 7/8inch
These shades are reproductions of lighting from the gas lighting era. These shades have a lip fitter that attaches with three screws or a straight fitter that will site on the fixture. Outside diameter 3 inches diameter fits to 3 1/4 inch opening and 4 inch opening fits to 3 7/8 inch diameter.
https://www.specialtylampshades.com/collections/3-and-4-inch-fitters
