马衔、马嚼子，俗称“水勒”，是放置在马嘴里的金属条状物，在某些情况下也可以由其它材料制成。马衔并不如大众所想象的安放在马的牙齿上，而是挂在前“切”齿之后和后“磨”齿之前的空当之中。这里通常叫做“bar”。俗话说一匹马“用它的牙夺过嚼子”（grab the bit in its teeth）实际上是指马用自己的嘴唇使劲夹住马衔而不听骑手的命令。依据马衔的设计和骑手的技术，马衔为骑手提供不同程度的对马的操控以及他和马匹之间的交流。因此，马衔的类型是否满足马的需要、是否能够安装到位从而正常工作以及是否能够尽量使马感觉舒适，都是十分重要的。

A bit is a type of horse tack used in equestrian activities, usually made of metal or a synthetic material, and is placed in the mouth of a horse or other equid and assists a rider in communicating with the animal. It rests on the bars of the mouth in an interdental region where there are no teeth. It is held on a horse's head by means of a bridle and has reins attached for use by a rider.