A martingale is any of several designs of tack that are used on horses to control head carriage. Martingales may be seen in a wide variety of equestrian disciplines, both riding and driving. Rules for their use vary widely; in some disciplines they are never used, others allow them for schooling but not in judged performance, and some organizations allow certain designs in competition.
The two most common types of martingale, the standing and the running, are used to control the horse's head height, and to prevent the horse from throwing its head so high that the rider loses control over the horse's speed, direction and shape of body. When a horse's head gets above a desired height, the martingale places pressure on the head so that it becomes more difficult or impossible to raise it higher.