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 gets hit in the face by the horse's or upper neck. 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.
The standing martingale, also known as a "tiedown" or a "head check," has a single strap which is attached to the girth, passes between the horse's front legs and is fixed to the back of the noseband. To prevent it from catching on other objects, it also has a neck strap. A variation is attached to a breastplate in lieu of a neck strap. When correctly fitted for English riding, it should be possible to push the martingale strap up to touch the horse's throatlatch.
A variation of the standing martingale, called a tiedown, is seen almost exclusively in the western riding disciplines. A tiedown is adjusted much shorter than a standing martingale and is intended primarily to prevent the horse from flipping its head up when asked to abruptly stop or turn in speed events. Users also claim that it gives the horse something to brace against for balance. It consists of an adjustable strap, one end which attaches to the horse's breastplate and the other which attaches to a noseband on the bridle. The noseband can be of leather, but may also be of lariat rope, or even plastic-covered cable, which can make the western tiedown considerably harsher than the English-style standing martingale. It is properly adjusted when it puts no pressure on the horse's nose when held at a normal position, but will immediately act if the horse raises its nose more than a few inches.
With both pieces of equipment, the slack is taken up out of the strap when the horse raises its head above the desired point, and pressure is placed on the horse's nose.
The standing martingale is competition legal for show hunter and hunt seat equitation riders over fences in the US, show jumping competitions in the UK, and is permissible and in common use in fox hunting, polocrosse, horseball, and polo. It is also seen on some military and police horses, partly for style and tradition, but also in the event of an emergency that may require the rider to handle the horse in an abrupt manner. It is not legal for flat classes. The tiedown is commonly seen in rodeo and speed events such as gymkhana games, but is not show legal in any other western-style horse show competition.