High Quality Content by WIKIPEDIA articles! The mouthpiece is the part of a horse`s bit that goes into the mouth of a horse, resting on the bars of the mouth in the sensitive interdental space where there are no teeth. The mouthpiece is possibly the most important determinant in the severity and action of the bit. Therefore, it should be carefully considered when choosing a bit for a horse. Some mouthpieces are not allowed in dressage competition.