A curb chain a linked chain or leather strap that runs under the chin groove of the horse from one cheekpiece to the other. It often has a "fly link" in the middle to apply a lip strap (used to keep the horse from grabbing the shank and to keep the curb chain from unfastening). A curb chain applies pressure to the chin groove when the curb rein of the bit is used. It also applies pressure to the bars of the mouth through the bit's mouthpiece, as it acts as a fulcrum. It should only tighten when the curb ring is rotated back 45 degrees.