The curb chain is an essential part of leverage bits like pelhams and Kimberwickes or double bridles. Adjusting them properly is key to having the bit work the way it is intended. Curb chain fit into the groove under a horse’s chin. When the rider pulls on the curb rein, it causes the shanks to rotate backward and the mouthpiece to move forward. As the shanks come back, the curb chain will come into contact with the horse’s chin and apply pressure. If the curb chain is too loose, the shanks will come back farther, magnifying the pressure; if the curb chain is too tight, the curb action is felt almost immediately.
The most common rule of thumb is for the curb to come into play when the shanks are rotated 45 degrees. On most bits that means that there should be about two fingers’ width of space between the curb chain and the curb groove.
Most of the time the curb chain is just that — a length of chain. In some cases, a leather strap is used. This is a milder alternative and you can skip the first step mentioned below.