A padlock is composed of a body, shackle, and locking mechanism. The typical shackle is a "U" shaped loop of metal ,round or square in cross-section that encompasses what is being secured by the padlock. Generally, most padlock shackles in older padlocks either swing away or slide out of the padlock body when in the unlocked position. Less common designs include a straight, circular shackle. Some shackles split apart and come together to lock and unlock.