A lampshade spider fitting connects a lampshade and lamp base together through the use of a harp, saddle and finial. Knowing if you require a lampshade with a spider fitting is important when purchasing a new shade for your lamp.
A lampshade spider consists of metal arms that attach to the inside top rim of the lampshade and join in the center to a hollow round metal disc that looks like a washer. Spider fittings are also called "washer fittings."
The harp is a "U" shaped piece of metal. The two ends of the harp slide into a saddle, which is a bracket attached to the socket of the lamp base.
The finial attaches the harp and the shade together. A screw attached to the top of the harp pushes through the round central piece of the spider, and the finial threads onto it, securing the shade to the harp.
If your lamp base has a harp and finial attached to it, or a saddle affixed to the socket of the lamp that the harp slides into, you will require a shade with a spider fitting.
Two additional types of lampshade fitters include clip fitters and UNO fitters. Both of these types allow the lampshade to attach to the lamp without the use of a harp and finial.