Finials are caps and ends to poles and posts. Garden posts, gate posts, bed posts and stair posts are all topped with a finial of one sort or another. Curtain poles are finished at each end with some sort of finial.
Balls are popular and unobtrusive, but there are as many finial designs and finishes as anyone cares to think up. Some are plain, simple and inexpensive, others require highly skilled craftsmanship, take many hours to make and are suitably costly.
Every pole catalogue has a specialist selection of finial designs and to a large extent it’s the finial you fall for that will determine the pole the rings, and any other window treatment detail.