A curb chain, or curb strap, is used on curb bits (pelhams,kimberwickes and Weymouths) when riding a horse. It is a linked chain 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.
The curb chain is used to control the action of the bit. Additionally, it also helps to keep the curb steady and in place within the mouth, and helps to hold a lip strap in place. The lip strap, although not always used, has three main functions: it helps prevent the horse from grabbing the shank (a habit that is most commonly seen in stallions), it helps prevent the curb chain from unfastening, and it helps to prevent the shanks on a sliding mouthpiece from accidentally flipping directions due to a sudden movement by the horse.
The curb chain applies pressure to the curb groove under a horse's chin when the curb rein of the bit is used. When the curb rein is pulled, the shank of the bit rotates back towards the chest of the horse and the cheek (upper shank) of the bit rotates forward (since it is a lever arm). The curb chain is attached to the rings at the end of the cheek, so, as the cheek moves forward, the chain is pulled and tightened in the curb groove. Once it comes in contact with the curb groove of the horse it acts as a fulcrum, causing the cannons of the bit mouthpiece to push down onto the horse's bars, thus amplifying the bit's pressure on the bars of the horse's mouth.

This double link stainless steel curb chain comes in one size only.