Solid restraints or furniture that are immobile and that can't be moved by the person who is restrained by them.
Examples include pillories, stocks mounted in place, shackles bolted or screwed to a surface, cuffs holding the subject inside a sex machine, and pipe arrangements built around the subject. Ropes, chains, and tentacles don't count.
The victim may have some freedom of movement or be bound by ropes or chains in addition to the stationary restraint, but the stationary restraint itself must be strong enough not to move or sway in response to the captive's struggling. The restraints themselves may be able to move, such as a machine capable of posing the victim, but the captive themselves should not be able to move the restraints.
This tag implicates restrained (learn more).
