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The bit shank is the side piece or cheekpiece of a curb bit, part of the bridle, used when riding on horses. The bit shank allows leverage to be added to the pressure of the rider's hands on the bit. Shanks are usually made of metal, may be straight or curved, and may be decorated in some disciplines. The headstall and curb chain or curb strap of the bridle is attached to the top of the shank, and the reins are attached at the bottom. Shanked curb bits are used in western riding for nearly all adult horses, and are seen in English riding disciplines primarily as part of the double bridle used by advanced dressage riders, and on the hybrid pelham bit that includes a ring for a second rein attached at the bit mouthpiece.
Direct pressure snaffle bits have no shanks, instead they have a single bit ring.
Bits that have shanks coming off the mouthpiece create leverage and place pressure on the poll via the crownpiece of the bridle, to the chin groove via the curb chain, and, especially with a "loose jaw" shank, may also touch the sides of the mouth and jaw. The shank and its leverage action is what defines a curb bit as a curb, regardless of mouthpiece. Though most curb bits have a solid mouthpiece, with or without a port, any bit with shanks and leverage is always a "curb" type bit, even if has a jointed mouthpiece. Shanked bits in the curb family include the Weymouth, which is the curb portion of the double bridle; the pelham bit, a single bit ridden with two sets of reins; and the single-reined curb bit.