The function of any curb is dependent on a properly adjusted curb chain or strap. Fitting into the chin groove and hooked to either side of the bit, the curb chain limits the amount of angle that the curb shanks can reach in relation to the mouth of the horse. When the rein is engaged, the lever action of the shanks puts some pressure on the poll of the horse through the cheek and headpiece, and on the mouth and jaw through essentially squeezing the lower jaw between the mouthpiece and the curb chain.

A tighter curb chain limits the amount of angle that the curb shanks can travel, thus causing less pressure to be put on the poll and jaw. However, this does mean that the action comes into play more immediately upon contacting the rein. Too tight a curb chain and the bit cannot function properly, while the lower jaw is also squeezed with the slightest of pressure on the reins. Too loose a curb chain and the lever action becomes more severe in the mouth, with the danger that the port will push up against the palate and essentially pry the horse’s mouth apart. The ideal point at which the curb chain should engage is when the cheeks are approximately at a 45 degree angle to the mouth of the horse.

Curb chains are typically double linked chains most of the way across, making them lay smoother in the chin groove. Single links on either end of the chain allow for ease of hooking. Single linked chains can also be found, but these are typically not as smooth. In either case, the chains must be twisted until all of the links lay flat. To soften a curb chain even more, a fleece, rubber, or leather curb guard can be put over the chains. Curb straps are also made to serve the same function, and they are typically made of leather, though they can also be found with fleece and elastic for greater comfort.