The piece of metal on a bridle that goes into a horse's mouth is called a bit. It is held in place by the bridle and connected to the reins, allowing the rider to communicate with the horse through pressure and release.
A bit does not work alone. Its effects are combined with signals from the rider's hands, legs, seat, and body position. Depending on its design, it may apply pressure to the tongue, bars of the mouth, lips, chin, or poll behind the horse's ears. Common types include snaffle bits and curb bits, each offering different leverage and levels of control.
A frequent mistake is calling the entire headgear a bit. The bridle is the complete arrangement of straps around the horse's head; the bit is only the mouthpiece, while the reins attach to its rings or shanks. Proper fitting matters because an unsuitable or harsh bit can cause pain, injury, or resistance. Some horses are ridden without a bit using a bitless bridle.