A curb chain required for proper use on any type of curb bit. It is a flat linked chain or flat strap that runs under the chin groove of the horse. It has a buckle or hook attachment and English designs have a “fly link” in the middle to hold a lip strap which helps keep the lip strap in place while the lip strap in turn prevents the curb chain from being lost if it becomes unhooked . On English bridles the horse is bridled with the curb chain undone on one side, then connected once on the horse.
The main use of the curb chain is to enhance and control the lever action of a curb bit. Additionally, it helps to keep the bit steady and in place within the mouth.
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.