Shackles, alternatively known as legcuffs, fetters, leg irons or ankle cuffs, are a form of restraint common around the world. Similar in nature to handcuffs, shackles behind a subject's ankles together. This prevents the subject from running, kicking, or otherwise moving at high speeds.
Police agencies around the world commonly use a modified form of shackles when transporting prisoners, who may attempt to escape.
A related restraint, mostly historically used, are bilboes: U-shaped leg irons with holes in the ends, through which a bolt was inserted.
In BDSM, shackles are a fairly common accessory, although many practitioners prefer to keep their subject's legs firmly together with tape. Shackles are designed to allow the subject to walk to some degree, which is sometimes unpopular in BDSM.