Circular barbells (horseshoe) are a curved piece of jewellery with two balls at the bottom. They are available in a range of sizes and typically worn in septum and lip piercings, but are also suitable for nostril and some ear piercings.
Circulars are available in different attachment styles depending on your preference:
Externally threaded - this is where the ball has a hole, and screws onto the circular.
Threadless - this is where the ball/top has a small pin on the bottom with a slight bend, and pushes into the circular, held in by tension.
Balls/tops must be the same attachment design as the circular to be compatible, eg. you cannot put a threadless top onto an externally threaded circular.
Different gauges (this refers to the thickness of the circular) are available to suit different piercings. The higher the number, the thinner the jewellery:
16g circulars are suitable for most septum, lip, ear and nose piercings.
14g circulars are suitable for some septum, lip and ear piercings for a bolder look.