Snaffle bit
A snaffle bit is the most common type of bit used while riding horses. It consists of a bit mouthpiece with a ring on either side and acts with direct pressure. A bridle utilizing only a snaffle bit is often called a "snaffle bridle", particularly in the English riding disciplines. A bridle that carries two bits, a curb bit and a snaffle, or "bradoon", is called a double bridle. A snaffle is not necessarily a bit with a jointed bit mouthpiece, as is often thought. A bit is a snaffle because it creates direct pressure without leverage on  the mouth. It is a bit without a shank. Therefore, a single- or double-jointed mouthpiece, though the most common designs for snaffle bits, does not make a bit a snaffle. Even a mullen mouth (a solid, slightly curved bar) or a bar bit is a snaffle. Action. The snaffle bit works on several parts of the horse's mouth; the mouthpiece of the bit acts on the tongue and bars, the lips of the horse also feel pressure from both the mouthpiece and the rings. The rings also serve to act on the side of the mouth, and, depending  on design, the sides of the jawbone. A snaffle is sometimes mistakenly thought of as "any mild bit". While direct pressure without leverage is milder than pressure with leverage, nonetheless, certain types of snaffle bits can be extremely harsh when manufactured with wire, twisted metal or other "sharp" elements. A thin or rough-surfaced snaffle, used harshly, can damage a horse's mouth. Curb chains or straps have no effect on a true snaffle because there is no leverage to act upon. English riders do not add any type of curb strap or curb chain to a snaffle bit. While  some riders in western disciplines do add a curb strap to the rings, it is merely a "hobble" for the rings, has no leverage effect and is there only as a safety feature to prevent the rings from being pulled through the mouth of the horse, should the animal gape open its mouth in an attempt to avoid the bit, an outcome prevented in an English bridle by the presence of a cavesson noseband. Difference from a curb. The snaffle differs from the pelham bit, the curb bit, and the kimberwicke in that it is a "non-leverage" bit, and  so does not amplify the pressure applied by the reins. With a snaffle, one ounce of pressure applied by the reins to a snaffle mouthpiece will apply one ounce of pressure on the mouth. With a curb, one ounce of pressure on the reins will apply more – sometimes far more – than one ounce of pressure on the horse's mouth. There are many riders (and a remarkable number of tack shops) who do not know the true definition of a snaffle: a bit that is non-leverage. This often results in a rider purchasing a jointed mouthpiece bit with shanks , because it is labeled a "snaffle," and believing that it is soft and kind because of the connotation the snaffle name has with being mild. In truth, the rider actually bought a curb bit with a jointed mouthpiece, which actually is a fairly severe bit due to the combination of a nutcracker effect on the jaw and leverage from the shanks. A true snaffle does not have a shank like a pelham or curb bit. Although the kimberwicke appears to have a D-shaped bit ring like a snaffle, the bit mouthpiece is not centered on the ring, and thus  applying the reins creates leverage; in the Uxeter kimberwicke, there are slots for the reins placed within the bit ring, which allows the reins to create additional leverage. Both are used with a curb chain, thus the ring acts like a bit shank and creates a slight amount of leverage, making it a type of curb bit. A true snaffle also will not be able to slide up and down the rings of the bit or cheekpieces of the bridle, as this would place it in the gag bit category. The mouthpiece. The mouthpiece is the more important part of  a snaffle, as it controls the severity of the bit. Thinner mouthpieces are more severe, as are those that are rougher. The snaffle rings. There are several types of rings that affect the action of the bit. Fitting. The most important thing to remember when fitting a bit is that no two horses are completely alike. What is preferred by one, may cause severe problems in another. It is therefore the rider's duty to find a bit that not only suits the horse (both mouthpiece and ring), but one that fits correctly. The three main criteria in fitting the  snaffle are the height the bit is raised in the mouth (adjusted by the cheekpieces), the width of the bit (from where the mouthpiece hits one ring, to where it hits the other), and the thickness of the mouthpiece. Height. Theories as to fitting the snaffle vary between horse owners, but the most common theory of fitting the snaffle is to adjust it so that it creates one or two wrinkles in the lips at the corner of the horse's mouth. The best way to determine how high a snaffle should be is to begin with the bit just  touching the corners of the horse's mouth, forming one wrinkle. If the rider holds the cheekpieces of the bridle and moves them up, there should remain enough give in the bridle to raise the bit in the horses mouth, however, there should not be excessive slack in the cheekpieces when this is done. The horse should keep its mouth closed over a properly-fitted bit (slight chewing is acceptable and a sign of relaxation) and hold its head quietly. A bit may need to be adjusted either higher or lower until the horse shows no signs of discomfort. The  height of the bit in the horse's mouth has little significant impact on its severity. Some riders mistakenly think that raising or lowering the bit increases its effect, but this is not correct. The bit is most effective when properly adjusted. Improper adjustment only causes discomfort, not increased control. Factors that affect the fit of the bit include both the length of the mouth overall, the length of the interdental space between the incisors and the molars where the bit rests on the bars (gums) of the horse's mouth, the thickness of the horse's tongue and the  height of the mouth from tongue to palate. There is less room for error with a horse who has a short mouth, thick tongue and a low palate than with a horse who has a longer mouth, thinner tongue and a higher palate. One of the important criteria when fitting the snaffle is that it does not hit the horse's teeth. The greater concern is that the bit not be so high as to constantly rub on the molars, which can cause considerable discomfort to the horse. A bit adjusted too low usually will not come anywhere near the  incisors, even on a short-mouthed horse, until the entire bridle is at risk of falling off. If the bit is adjusted too low (not touching the corner of the mouth), it is primarily a safety concern, though the action of the bit can also be altered and lead to discomfort. A horse can get its tongue over a too-low bit and thus evade its pressure, plus the action of the bit is altered and it will not act on the mouth as it was designed. Horses with a bit too low will often open their mouths to evade  pressure and may chew on it excessively. In extreme cases, the bridle could even fall off if the rider pulls hard on the reins, hence raising the bit and loosening the cheekpieces, at the same time the horse rubs, tosses or shakes its head vigorously. Many horses will "carry" a too-low bit themselves, using their tongue to hold it in the proper place. Some trainers, especially in western riding disciplines, consider this desirable and adjust a bridle a bit low to encourage this behavior. Other trainers, especially in English riding disciplines, prefer to hang the bit a little higher  so it is in the correct position without need for the horse to move it there. If the bit is too high (depending on the horse, at three or more wrinkles in the lips), it will irritate the lips, leading to callousing and a loss of sensitivity over time. However, the more immediate consequence is that the horse feels constant bit pressure and cannot get any release, even if the rider loosens the reins. This leads to the horse becoming tense in the jaw and resisting the bit. Most of all, if a too-high bit rubs on the  molars, this discomfort will cause the horse to toss its head and otherwise express its displeasure at the situation, leading to a poor performance. If the horse tosses its head or attempts to evade contact with a bit, improper fit is usually the cause, but other factors should be considered. A rider needs to verify with a veterinarian that the horse does not have a dental problem. Then bit fit and the type of bit needs to be considered. But finally, the skills of the rider may be a factor. Even the gentlest bit properly adjusted may still cause discomfort  to a horse in the hands of a poor rider. Width. The snaffle should generally be no more than  inch wider than the horse's mouth. A horse's mouth can be measured by placing a wooden dowel or a piece of string into the mouth where the bit will go and marking it at the edges of the horse's lips. A bit that is too narrow can cause pinching (which may be very severe in a loose ring), and the pinching may lead to behavior problems when the horse experiences the discomfort. A pinching bit will also cause  callousing on the lips. The lesser sin is a bit that is too wide, which does not pinch the lips, but does not allow for effective communication between horse and rider. The nutcracker effect of a jointed snaffle presents a fit issue as well; the joint of a too-wide mouthpiece will hit the roof of the horse's mouth when the reins are tightened. Mouthpiece diameter. Competition rules require bits to have a minimum diameter, but have no upper limits on thickness. Many horsepeople believe that a fatter mouthpiece is always a milder mouthpiece, because thin mouthpieces localize the  pressure on the bars of the mouth. However, the horse's mouth is filled almost completely by his tongue. Therefore, many horses (especially those with large, fleshy tongues) prefer an average diameter mouthpiece, which provides slightly more space in an already cramped mouth. Additionally, thicker mouthpieces do not give a great deal of extra bearing surface, and so generally do not help as much as many riders believe. To make a bit milder, it can be wrapped with rubber or made of a softer plastic material instead of metal. However, mouthpieces that are extremely thin, such as wire mouthpieces or  those that are only –" in thickness, are never mild. These can be damaging to a horse's mouth. Citations. Edwards Complete Book of Bits and Bitting pp. 52–54 Kapitzke Bit and Reins p. 79 Edwards Complete Book of Bits and Bitting p. 55 Edwards Complete Book of Bits and Bitting p. 68 Edwards Complete Book of Bits and Bitting p. 57 Edwards Complete Book of Bits and Bitting p. 58 Kapitzke Bit and Reins p. 95