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She who dies with the most bits wins! Or so it sometimes seems, especially when we take a good, hard look in our tack rooms. After all, who among us isn't guilty of using a trial and error method when it comes to searching for that perfect bit for our horses? And it's not totally our fault. Browse through any tack catalog or on-line store and you'll find literally hundreds of types and styles of bits - and virtually no direction as to how each one differs in the effect it has on our horses. So instead of wondering if a thousand years from now archeologists unearthing your tack room would pin you as an avid bit collector, let's take a more scientific approach to bit selection.

Exactly how bits function is directly related to their physical properties. Differences in size, weight, and shape alter a bit's intensity, mode of action and point of engagement. In addition, horses have anatomical diversities that alter the shape and size of their skulls, the soft tissue of their face and mouth, and their teeth placement. These differences can change the way a particular bit acts upon individual horses. Therefore, no single bit is perfect for every horse. Our job as equestrians is to find those bits that provide the best means of communicating with our horses, while conforming to our horse's unique anatomical structure. Doing so will not only save us time and money in the long run, but will make us better riders.

Bits function by applying pressure to sensitive areas of the horse's mouth, lips and head. There are seven basic pressure points that are used by modern bits. Bits vary in both the number and location of the pressure points they use, as well as the intensity of the pressure they apply. To understand how bits work, let's take a quick look at these pressure points:

Leverage bits, which include the various types of kimberwicks, pelhams and curb bits, amplify the force or amount of pressure applied by the rider. The amount of amplification is determined, in part, by the ratio of the length of the bit's shank vs. cheek (upper shank) length. As this ratio increases, so does the bit's ability to amplify pressure. Thus, a bit that has a four inch shank and a two inch cheek has a 2:1 ratio and will exert twice the pressure to the horse's mouth that the rider exerts on the reins. A bit with a 4:1 ratio will quadruple the pressure. So if the rider applies three ounces of pressure to the reins, the horse's mouth will receive 12 ounces.

Other elements of bit design also affect how a bit functions. Bits with moving parts, such as loose rings on a snaffle or a broken mouthpiece, allow a certain amount of rotation to occur before bit pressure is applied. These bits increase the signal by giving the horse a preparatory warning that bit pressure is forthcoming. Eggbutt and D-ring snaffles afford less signal than a ring snaffle because the former has rings that are permanently attached to the bit. However, this type of attachment applies more pressure to the horse's lips, making him easier to control and turn in the desired direction.

On curb bits, both the angle and length of shank affect the signal or speed of communication. While the ratio between the shank and the cheek determines the amount of amplification that is possible with a particular bit, a longer shank must rotate more than a shorter one before pressure is applied to the horse's mouth. This increases the signal by giving the horse more time to respond. Likewise, a curved shank must also rotate more than a straight one before pressure is felt, so a curb bit with straight sides is quicker-acting than one that is angled. In addition, an angled shank can't travel as far as a straight one, thus it's potential to amplify pressure is reduced.

Lastly, the adjustment of the curb strap can also alter the speed of communication and the intensity of the signal. A chin strap that is longer or more loosely attached allows the curb bit to rotate farther before leverage action occurs. While pressure on the poll is exerted as the bit rotates, the intensity of the pressure on the tongue, bars and chin groove is not felt until the chin strap is pulled tight against the lower jaw. Likewise, curb straps used with high-port bits, like a spade, are generally adjusted tighter to prevent the port from damaging the roof of the horse's mouth. 

This same effect is seen with unbroken bits. The curvature of a mullen mouthpiece places the pressure primarily on the tongue and lips. A mullen mouthpiece is ideal for horses that have sensitive bars. Similarly, the primary purpose of a port is to provide pressure relief to the tongue. As pressure is applied and the bit rotates backwards, the horse's tongue moves into the opening of the port. The pressure is then focused on the bars of the mouth. This will be negated if the mouthpiece contains a roller or other device that blocks the opening to the port.

Higher ports, like those seen on a spade bit, are also used to apply pressure to the horse's palate. This pressure is alleviated when the horse lowers his head and flexes at the poll. For most horses, the port must be at least 2 to 2/12 inches high in order to make contact with their palate. However, as horses age and the slope of their incisors increases, the distance from their tongue to their palate also decreases.

Since spade bits are considered severe, they should only be used by experienced equestrians with educated hands. But the truth is, any bit is capable of causing undue pain in the hands of an inconsiderate or inexperienced rider. So regardless of the bit you choose, the rider's hands are the ultimate factor that determines a bit's severity.


  • Chin groove:  This area is where the curb strap or chain lies across the lower jaw bones. Due to the skin's close proximity to the bone, the chin groove is also sensitive to pressure.

  • Poll:  When rein pressure is applied to leverage-type bits, the crown or headpiece of the bridle can be pulled tighter to exert pressure on the horse's poll. From the time we first halter break our horses, we teach them to give to this pressure. During riding, pressure exerted on the poll encourages a horse to lower its head.

  • Nose:  Like many pressure points, the skin over the nose is in close contact with bone. This area of pressure is used primarily by bosals and hackamores.

  • Lower jaw:  Hackamores and bosals also apply pressure to the lower jaw, which is another area where thin skin covers bone.

A Look at How Science Influences Riding


In addition to the tongue and bars, there are five external pressure points that feel the effects of the bit.

Due to their design, bits either exert direct pressure or a leverage effect when rein pressure is applied. Bits that exert direct pressure are called snaffles, but this nomenclature is sometimes used for any bit that has a broken or jointed mouthpiece. A true snaffle doesn't have cheekpieces, shanks or require a curb strap. Examples of a true snaffle include ring snaffles, eggbutt snaffles and D-ring snaffles. These bits exert the exact amount of pressure to the mouth that the rider has applied to the reins. So if the rider exhibits a force equal to three ounces of pressure, the horse feels three ounces of pressure.

A comparison of a true snaffle and a leverage bit.

Top bit - Eggbutt snaffle

Bottom bit - Tom Thumb

One of the most influential aspects of a bit's severity is the mouthpiece. Changes in the thickness, smoothness, and shape of the mouthpiece alters how much and where pressure is applied.

As a general rule, the diameter of a mouthpiece is inversely related to its severity. So the larger the diameter of the mouthpiece, the more contact it makes with the tongue and the milder it is. This principle can be demonstrated by pressing the length of your finger across your arm, then poking the tip of your finger into your arm. The more concentrated the pressure, the more intense it feels. Rubber coated bits, due to their larger diameter and soft material, tend to be the mildest mouthpieces.

In terms of size, bitting issues are more common with wider, rather than narrow mouthpieces. Since bits with a larger diameter mouthpiece are generally milder, these bits are often used for breaking young horses. However, a horse's mouth doesn't reach full maturity until five years of age so a two-year-old's mouth may not be big enough to comfortably carry a larger mouthpiece. For the same reason, horses with thicker tongues may find that a thick mouthpiece places too much pressure on their tongue when their mouth is closed. Gaping of the mouth, even in absence of rein pressure, can be indicative that the mouthpiece is too thick for that particular horse.

This same principle also applies to the smoothness of the mouthpiece. The less bumps and lumps molded into the mouthpiece, the more evenly the pressure is spread across the tongue and bars. A twisted wire or chain mouthpiece concentrates rein pressure into a few select spots. This is also illustrated when comparing a french link mouthpiece with a Dr. Bristol. Both have a smooth, flat center link which gives these mouthpieces two breaks. However, the french link lies flat against the tongue, while the Dr. Bristol link makes contact at a 45% angle. The Dr. Bristol is a more severe bit because the link is positioned so the edge of the link, rather than the smooth flat section, makes contact with the tongue.

Because very narrow, twisted wire, chain or Dr. Bristol mouthpieces are relatively severe, these types of bits are banned from some types of competitions. In addition, combining these mouthpieces with a leverage-type bit produces a bit that is much more severe than when these mouthpieces are used with a direct-pressure snaffle.

The shape of the mouthpiece and its ability to conform to the curvature of the tongue influences severity as this determines where the bit will apply pressure. A single jointed mouthpiece will rise in the center and squeeze the sides of the tongue and bars when pressure is placed on the reins. Since these areas are more sensitive than the center of the tongue, a single jointed mouthpiece is felt more strongly than a comparable one with two or more joints. 

  • Tongue:  All bits that have a mouthpiece exert pressure on the tongue. Depending upon the size and shape of the mouthpiece, this pressure can be spread out evenly across the tongue or concentrated on the sides. The sides of a horse's tongue are more sensitive than the center.

  • Bars:  This is the interdental space on the horse's lower mandible that lies directly in front of the first premolars. Since only a thin layer of epidermis covers the bone here, this area is highly sensitive. Horses also differ in the shape and smoothness of their bars.

  • Lips:  Although a horse's lips are composed of soft tissue, they are also covered by a thin layer of skin. This makes the lips sensitive to pressure, especially at the corners of the mouth.

The curved shanks of this curb bit limit the amount of leverage this bit can exert. The port in the mouthpiece directs the rein pressure away from the tongue and onto the bars.

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