The American spider fitter is intended for use with a harp and finial. If your lamp has a harp, you will need a shade with a spider fitter. The spider fitter sits on top of the harp, is secured by a finial.The hole opening is 1/2 inch.
The European fitter fits most Ikea lamps and pendant cord sets. No harp is needed. The shade attaches directly to the lamp socket and is tightened with a ring. The hole opening is 1 5/8 inch.