The bit is an important item of a horse's tack. It usually refers to the assembly of components that contacts and controls the horses mouth, and includes the shanks, rings, cheekpads and mullen, all described here below, but it also sometimes simply refers to the mullen, the piece that fits inside the horses mouth. The mullen extends across the horses mouth and rests on the bars, the region between the incisors and molars where there are no teeth. The bit is located on the horse's head by the headstall, and which has itself several components to allow the most comfortable adjustment of bit location and control.
The bit, bridle and reins function together to give control of the horse's head to the rider. The bit applies pressure to the horse's mouth, and reinforces the other control signals from the rider's legs and weight distribution. A well schooled horse needs little pressure on the bit from a skilled rider. Studies have indicated that soft, consistent bit contact between the rider and horse causes the animal less stress than intermittent or unpredictable contact.
Some bits combine both direct pressure and leverage, the most common examples being the Pelham bit, which has shanks and rings allowing both direct and leverage pressure on a single bit and is ridden with four reins;Pelham bit, which has shanks and rings allowing both direct and leverage pressure on a single bit and is ridden with four reins; the Kimblewick or Kimberwicke, a hybrid bit that uses minimal leverage on a modified snaffle-type ring combined with a mouthpiece that is usually seen more often on curb bits, ridden with two reins; and the double bridle, which places a curb and a snaffle bit simultaneously in the horse's mouth so that each may act independently of the other, ridden with four reins. Another bit that combines direct pressure and leverage in a unique manner is the Gag bit, a bit derived from the snaffle that, instead of having a rein attached to the mouthpiece, runs the rein through a set of rings that attach directly to the headstall, creating extra pressure on the lips and poll when applied. Usually used for correction of specific problems, the gag bit is generally illegal in the show ring and racecourse.
Bits and the behavior of horses while wearing bits have made their way into popular culture outside of the horse world.