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.