Safety chains

Safety chains are two steel chains crossed in an X pattern below the trailer coupler, connecting the trailer frame to the tow vehicle. If the coupler fails, the chains catch the tongue before it hits the road and let the driver bring the rig to a controlled stop.

Chains are crossed so a falling tongue rests on the X, not on the pavement. Length should allow full-lock turning without binding but not drag on the ground. Chain class is rated to trailer weight: Class III, for example, covers 5,000–7,500 lb (2,270–3,400 kg). Required by virtually every US state's towing regulations.