A bail is a component of certain types of jewelry, mostly necklace

Necklace

A necklace is an article of jewellery which is worn around the neck. Necklaces are frequently formed from a metal jewellery chain. Others are woven or manufactured from cloth using string or twine....

s, that is used to attach a pendant

Pendant

A pendant is a loose-hanging piece of jewellery, generally attached by a small loop to a necklace, when the ensemble may be known as a "pendant necklace". A pendant earring is an earring with a piece hanging down. In modern French "pendant" is the gerund form of “hanging”...

or stone. The bail is normally placed in the center of the necklace where the pendant hangs.

Some bails are made so a pendant can be attached after the necklace production is completed. This way, a necklace design can be mass-produced for multiple companies and the pendants can be attached after the necklaces are shipped to them.