Rigid Irons are a type of restraint used in bondage that keeps the limbs (and sometimes the neck) locked at a fixed distance to each other. Rigid irons allow for far less movement than handcuffs, which have a flexible link. There are several configurations of rigid irons, such from simply connecting two limbs on either side of the body (wrist to wrist or ankle to ankle), to more complex configurations that lock all limbs (ankle to wrist to wrist to ankle).
The lack of flexibility in the binds means that rigid iron can become uncomfortable and even painful after some time. It is recommended not to lock them directly on the skin, but rather over gloves and boots and so on