Reins are items of horse tack

Horse tack

Tack is a term used to describe any of the various equipment and accessories worn by horses in the course of their use as domesticated animals. Saddles, stirrups, bridles, halters, reins, bits, harnesses, martingales, and breastplates are all forms of horse tack...

, used to direct a horse

Horse

The horse is one of two extant subspecies of Equus ferus, or the wild horse. It is a single-hooved mammal belonging to the taxonomic family Equidae. The horse has evolved over the past 45 to 55 million years from a small multi-toed creature into the large, single-toed animal of today...

 or other animal used for riding or driving

Driving

Driving is the controlled operation and movement of a land vehicle, such as a car, truck or bus.Although direct operation of a bicycle and a mounted animal are commonly referred to as riding, such operators are legally considered drivers and are required to obey the rules of the road...

. Reins can be made of leather, nylon, metal, or other materials, and attach to a bridle

Bridle

A bridle is a piece of equipment used to direct a horse. As defined in the Oxford English Dictionary, the "bridle" includes both the headstall that holds a bit that goes in the mouth of a horse, and the reins that are attached to the bit....

 via either its bit

Bit (horse)

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...

 or its noseband

Noseband

A noseband is the part of a horse's bridle that encircles the nose and jaw of the horse. In English riding, where the noseband is separately attached to its own headstall or crownpiece, held independently of the bit, it is often called a cavesson or caveson noseband...

.

Use for riding

Reins are used to give subtle commands or cues, also known as rein aids

Riding aids

Riding aids are the cues a rider gives to a horse to communicate what they want the animal to do. Riding aids are broken into the natural aids and the artificial aids.-Natural aids:...

. Various commands may signal a turn, ask for a slower speed, request a halt or rein back. Rein aids are used along with leg aids, shifting of body weight, and sometimes voice commands.

Harness reins

On some types of harnesses

Horse harness

A horse harness is a type of horse tack that allows a horse or other equine to pull various horse-drawn vehicles such as a carriage, wagon or sleigh. Harnesses may also be used to hitch animals to other loads such as a plow or canal boat....

 there might be supporting rings or "terrets" used to carry the reins over the animal's back. When pairs of equines are used in drawing a wagon

Wagon

A wagon is a heavy four-wheeled vehicle pulled by draught animals; it was formerly often called a wain, and if low and sideless may be called a dray, trolley or float....

 or coach

Coach (vehicle)

A coach is a large motor vehicle, a type of bus, used for conveying passengers on excursions and on longer distance express coach scheduled transport between cities - or even between countries...

 it is usual for the outer side of each pair to be connected to the reins and for the inside of the bits to be connected between the pair of horses by a short bridging strap or rope. The driver carries "four-in-hand" or "six-in-hand" being the number of reins connecting to the pairs.

Other uses

A single rein or rope may be attached to a halter

Halter

A halter, headcollar, or, less often, headstall, is headgear that is used to lead or tie up livestock and, occasionally, other animals; it fits behind the ears , and around the muzzle. To handle the animal, usually a lead rope or lead shank is attached...

 to lead or guide a horse or packhorse

Packhorse

.A packhorse or pack horse refers generally to an equid such as a horse, mule, donkey or pony used for carrying goods on their backs, usually carried in sidebags or panniers. Typically packhorses are used to cross difficult terrain, where the absence of roads prevents the use of wheeled vehicles. ...

. A long rein called a longe

Longeing

Longeing or lungeing is a technique for training horses, where a horse is asked to work at the end of a long line and respond to commands from a handler on the ground who holds the line. It is also a critical component of the sport of equestrian vaulting...

 line may be used to allow the horse to move in a circle for training

Horse training

Horse training refers to a variety of practices that teach horses to perform certain behaviors when asked to do so by humans. Horses are trained to be manageable by humans for everyday care as well as for equestrian activities from horse racing to therapeutic horseback riding for people with...

 purposes, or for the purpose of a clinical lameness evaluation by a veterinarian. On certain designs of headgear, a third rein may be added to the paired reins, used for leading, longeing, or other specialized or stylistic purposes. The best-known example of a third rein used in the USA is the leading rein of the mecate of the classic bosal hackamore

Hackamore

A hackamore is a type of animal headgear which does not have a bit. Instead, it has a special type of noseband that works on pressure points on the face, nose, and chin...

.

Types

Types of reins include:

• Closed reins — (also called Loop reins) reins that are either a single piece or that buckle together at the ends. English riders
  English riding
  English riding is a term used to describe a form of horse riding that is seen throughout the world. There are many variations in English riding, but all feature a flat English saddle without the deep seat, high cantle or saddle horn seen on a Western saddle nor the knee pads seen on an Australian...
  
   usually use closed reins. Western riders
  Western riding
  Western riding is a style of horseback riding which evolved from the ranching and warfare traditions brought to the Americas by the Spanish Conquistadors, and both equipment and riding style evolved to meet the working needs of the cowboy in the American West...
  
   in timed rodeo
  Rodeo
  Rodeo is a competitive sport which arose out of the working practices of cattle herding in Spain, Mexico, and later the United States, Canada, South America and Australia. It was based on the skills required of the working vaqueros and later, cowboys, in what today is the western United States,...
  
   events use a single closed rein, as do those who use a romal
  Romal
  A Romal , is a type of long quirt attached to the end of a set of closed reins that are connected to the bridle of a horse. It is not to be used to strike a horse, but rather was a tool used to assist in moving cattle....
  
  . A closed rein helps prevent the rider from dropping the reins.
• Split reins — a rein style seen in western riding
  Western riding
  Western riding is a style of horseback riding which evolved from the ranching and warfare traditions brought to the Americas by the Spanish Conquistadors, and both equipment and riding style evolved to meet the working needs of the cowboy in the American West...
  
   where the reins are not attached to one another at the ends. They prevent a horse from tangling its feet in a looped rein, particularly when the rider is dismounted. They are considerably longer than closed reins.
• Side reins
  Side reins
  Side reins are equipment used when longeing a horse, running from the bit of the bridle to the saddle or surcingle. As a horse training tool, they encourage flexion and softness in the horse's mouth. For longe work with a rider up who does not carry ordinary riding reins, they help calm and...
  
   — used when longeing
  Longeing
  Longeing or lungeing is a technique for training horses, where a horse is asked to work at the end of a long line and respond to commands from a handler on the ground who holds the line. It is also a critical component of the sport of equestrian vaulting...
  
   a horse, attached from the bit to the saddle or surcingle
  Surcingle
  A surcingle is a strap made of leather or leather-like synthetic materials such as nylon or neoprene, sometimes with elastic, that fastens around a horse's girth area. A surcingle may be used for ground training, some types of in-hand exhibition, and over a saddle or horse pack to stabilize the load...
  
  , they are not meant to be held by the rider.
• Long reins, longlines, or driving lines — exceptionally long reins which allow the rider to control the horse from a cart
  Cart
  A cart is a vehicle designed for transport, using two wheels and normally pulled by one or a pair of draught animals. A handcart is pulled or pushed by one or more people...
  
  , or from the ground, with the handler walking behind the horse.
• Romal
  Romal
  A Romal , is a type of long quirt attached to the end of a set of closed reins that are connected to the bridle of a horse. It is not to be used to strike a horse, but rather was a tool used to assist in moving cattle....
  
   reins — a rein style from the vaquero
  Cowboy
  A cowboy is an animal herder who tends cattle on ranches in North America, traditionally on horseback, and often performs a multitude of other ranch-related tasks. The historic American cowboy of the late 19th century arose from the vaquero traditions of northern Mexico and became a figure of...
  
   tradition that incorporates a closed rein with a long quirt
  Quirt
  A quirt is a forked type of stock whip which usually has two falls at the end . Sometimes called a riding quirt, horse quirt, or a dog quirt.The falls on a quirt are made of leather, buffalo, or cow hide...
  
   at the end.

• Mecate
  Mecate (rein)
  The mecate is the rein system of the bosal style hackamore used to train young horses. It is a long rope, traditionally of horsehair, approximately 20-25 feet long and up to about 3/4 inch in diameter...
  
   — a style of rein seen on a bosal
  Bosal
  A bosal is a type of noseband used on the classic hackamore of the vaquero tradition. It is usually made of braided rawhide and is fitted to the horse in a manner that allows it to rest quietly until the rider uses the reins to give a signal. It acts upon the horse's nose and jaw...
  
   style hackamore
  Hackamore
  A hackamore is a type of animal headgear which does not have a bit. Instead, it has a special type of noseband that works on pressure points on the face, nose, and chin...
  
   made of a single piece of rope that encompasses both a closed rein and a leading rope.

• Reins used on bridles with two reins:
  • Snaffle rein — Usually a laced rein that buckles at the center, used on the bradoon of a double bridle
    Double bridle
    A double bridle, also called a full bridle or Weymouth bridle, is a bridle that has two bits and four reins . One bit is the bradoon , is a modified snaffle bit that is smaller in diameter and has smaller bit rings than a traditional snaffle, and it is adjusted so that it sits above and behind the...
    
    , or the upper ring of a pelham bit
    Pelham bit
    A pelham bit is a type of bit used when riding a horse. It has elements of both a curb bit and a snaffle bit. In this respect a pelham bit functions similar to a double bridle, and like a double bridle it normally has "double" reins: a set of curb reins and a set of snaffle reins. Because it has...
    
    .
  • Curb rein: The rein used at the end of the shank of a curb bit
    Curb bit
    A curb bit is a type of bit used for riding horses that uses lever action. It includes the pelham bit and the Weymouth curb along with the traditional "curb bit" used mainly by Western riders....
    
     or pelham. Modern curb reins usually buckle together at the ends, though reins of the classical curb were sewn together at the ends to create a single rein.
• Draw reins and running reins
  Draw reins and running reins
  Draw reins and running reins are pieces of riding equipment used for training that use the Mechanical advantage of a 'single movable pulley' to cause the horse to bring its head down and inward...
  
   — long reins, usually made of leather or nylon webbing, that attach to the saddle or the girth, run through the bit ring
  Bit ring
  The bit ring is the ring on the side of a horse's bit, particularly on a snaffle bit. It is used as a point of attachment for the cheekpieces of the bridle and for the reins. It also has an effect on the action of the bit...
  
  s, and back to the rider. Several design variations, they add mechanical advantage
  Mechanical advantage
  Mechanical advantage is a measure of the force amplification achieved by using a tool, mechanical device or machine system. Ideally, the device preserves the input power and simply trades off forces against movement to obtain a desired amplification in the output force...
  
   to the rider's hands and may the horse's ability to raise its head. Often used in conjunction with a snaffle rein by English riders, usually used alone by western riders.
• Lead rein — A third rein used on bridles, not to be confused with the single lead rope of a halter
  Halter
  A halter, headcollar, or, less often, headstall, is headgear that is used to lead or tie up livestock and, occasionally, other animals; it fits behind the ears , and around the muzzle. To handle the animal, usually a lead rope or lead shank is attached...
  
   nor the direct rein aid
  Riding aids
  Riding aids are the cues a rider gives to a horse to communicate what they want the animal to do. Riding aids are broken into the natural aids and the artificial aids.-Natural aids:...
  
   known as the "leading rein." In North America
  North America
  North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...
  
  a third rein is most commonly seen as part of the mecate of a hackamore
  Hackamore
  A hackamore is a type of animal headgear which does not have a bit. Instead, it has a special type of noseband that works on pressure points on the face, nose, and chin...
  
  . In Mongolia
  Mongolia
  Mongolia is a landlocked country in East and Central Asia. It is bordered by Russia to the north and China to the south, east and west. Although Mongolia does not share a border with Kazakhstan, its western-most point is only from Kazakhstan's eastern tip. Ulan Bator, the capital and largest...
  
   it is integral to the bridle, and tied to either a bit ring
  Bit ring
  The bit ring is the ring on the side of a horse's bit, particularly on a snaffle bit. It is used as a point of attachment for the cheekpieces of the bridle and for the reins. It also has an effect on the action of the bit...
  
   or a chin strap
  Curb chain
  A curb chain, or curb strap, is a piece of horse tack used on any type of curb bit. It is a flat linked chain or strap that runs under the chin groove of the horse, between the bit shank purchase arms. It has a buckle or hook attachment and often has a "fly link" in the middle to apply a lip strap...
  
  .
• Double reins—The combined use of two pairs of reins, a curb rein and a snaffle rein. This is usually two single (buckled or sewn) reins, though sometimes split reins may be seen on western-style bridles. Double reins are used with a double bridle
  Double bridle
  A double bridle, also called a full bridle or Weymouth bridle, is a bridle that has two bits and four reins . One bit is the bradoon , is a modified snaffle bit that is smaller in diameter and has smaller bit rings than a traditional snaffle, and it is adjusted so that it sits above and behind the...
  
  , with bits such as the Pelham bit
  Pelham bit
  A pelham bit is a type of bit used when riding a horse. It has elements of both a curb bit and a snaffle bit. In this respect a pelham bit functions similar to a double bridle, and like a double bridle it normally has "double" reins: a set of curb reins and a set of snaffle reins. Because it has...
  
   and, less often, on some gag bit
  Gag bit
  The gag bit is a type of bit for a horse. With two sets of reins, the gag bit gives a rider the ability to use either a standard direct action or a gag action. In this, the gag bit is related to a Pelham bit and a double bridle...
  
  s used for polo
  Polo
  Polo is a team sport played on horseback in which the objective is to score goals against an opposing team. Sometimes called, "The Sport of Kings", it was highly popularized by the British. Players score by driving a small white plastic or wooden ball into the opposing team's goal using a...
  
  .

In popular expression

In popular culture, to rein in means to hold back, slow down, control or limit. Sometimes the eggcorn

Eggcorn

In linguistics, an eggcorn is an idiosyncratic substitution of a word or phrase for a word or words that sound similar or identical in the speaker's dialect. The new phrase introduces a meaning that is different from the original, but plausible in the same context, such as "old-timers' disease" for...

, reign

Reign

A reign is the term used to describe the period of a person's or dynasty's occupation of the office of monarch of a nation or of a people . In most hereditary monarchies and some elective monarchies A reign is the term used to describe the period of a person's or dynasty's occupation of the office...

in, is used.
Wide usage of the opposing free rein can be dated to the early 1950s, and means to give or allow complete freedom, in action and decision over something.