Bits are required in many competitive disciplines. With so many types to choose from, how do you select the best option for your horse? Let’s look at bit action and its effects on the horse’s mouth.

Bits are mandatory in many types of competition, and every rider is faced with the challenge of selecting the right one for his or her horse. Because the bit contacts sensitive structures in the horse’s mouth, improper selection, fitting or use is likely to cause resistance to rein action, or even injury to the mouth. Areas where the bit crosses a bony surface, such as the hard palate and the bars of the mouth, are particularly vulnerable to painful pressure from the bit.

Taking Away Guesswork

Tack stores carry a wide range of bits – some familiar, others novel and different (and not all legal for use in recognized competition). In recent years, bits have been designed to make the horse more comfortable, rather than as a means of increasing rider control, which is certainly a positive step. Unfortunately, the information needed to help riders choose appropriate bits is still limited.

In an effort to introduce some science into the art of bitting and bit selection, researchers at the McPhail Center have completed a series of studies on equine oral conformation and bitting. In this article, I’ll describe the position of various bits in the horse’s mouth and their relationship to sensitive structures. In Part 2 of this article (next issue), we’ll explore the movements of the bit within the oral cavity.

Oral Anatomy and Conformation

The bit rests on the tongue and the gums overlying the bars of the mandibles. The hard palate, which forms the roof of the mouth, marks the upper boundary of the oral cavity. The tongue normally fills the oral cavity, and the bit is interposed between the soft muscular tongue and the hard bony palate. When the horse accepts the bit, the muscles of the tongue relax, allowing it to be indented by the bit, thereby relieving bit pressure against the palate. The tongue usually covers the bars and protects them from direct bit pressure.

Horses differ in the size and shape of their oral cavities, and these differences dictate the mouthpiece types and sizes that can be accommodated comfortably. Factors to consider include:

•The position of the corners of the lips relative to the bars
• The width across the jaw between the corners of the lips on the left and right sides
• The shape of the palate (flat or arched)
• The thickness and width of the tongue

The first part of our study used x-rays to measure the internal dimensions of the horse’s oral cavity. The goal was to determine whether mouth size was proportional to horse size. The subjects were four Warmbloods and four Thoroughbreds. The height and length of each horse’s oral cavity were measured radiographically, and these dimensions were correlated with the horse’s height at the withers. The results showed no relationship between a horse’s height and the size of his oral cavity. This means we cannot assume a large horse should wear a bigger bit than a small horse.

A Look at Bit Position

Bits vary in size, shape and mechanics of action. Most riders are aware that individual horses respond better to certain bits, and that these preferences may be related to differences in oral conformation or the horse’s sensitivity to the mechanism of action. Our subjective judgment of the horse’s response is the main criterion used to select an appropriate bit for him.

The goals of our second study were to describe the position of different types of bits inside the horse’s mouth, and to measure their proximity to his hard palate and premolar teeth, both of which are readily identified on radiographs. The eight horses from the previous study were fitted with a snaffle bridle that was adjusted to produce two small wrinkles at the corners of the lips. A flash noseband was fitted snugly, but not tightly enough to indent the skin.

We studied four bits. The width of each mouthpiece was equal to or up to 0.5 cm (about ¼”) wider than the horse’s mouth, and the mouthpieces were the same thickness at the point where they crossed the bars of the mouth. We took two radiographs each of the four bits: one with the reins loose, and the other with equal tension applied to both reins.

1. Loose-Ring Jointed Snaffle:

The single-jointed bit (right) was positioned with the central joint and the arms of the mouthpieces hanging down toward the incisor teeth, with the mouthpiece rotated around the ring to the four o’clock position rather than being in the middle of the ring at three o’clock. The sharply curved profile of the joint in the middle of the mouthpiece protruded toward the horse’s palate. The loose ring allowed the mouthpiece to rotate freely, so the horse could use his tongue to move it into different positions, perhaps as a means of changing the areas under pressure.

When tension was applied to the reins, the mouthpiece pressed more deeply into the tongue, causing the joint to move away from the palate. Single-jointed bits are usually described as having a nutcracker-like action, the implication being that when tension is applied to the reins, the angle between the arms of the mouthpiece closes with the risk of the joint being pushed toward the palate. In our study, any nutcracker effect that tended to push the joint toward the palate was offset by more tongue indentation.

2. Baucher:

This snaffle also has a single joint, but its mechanics are different from the loose-ring snaffle’s because of the way it attaches to the bridle cheekpieces and reins. With a Baucher, the cheekpiece attaches to a small upper ring that does not allow the mouthpiece to rotate around it. Consequently, the mouthpiece is fixed in a more elevated position on the horse’s tongue than the loose-ring snaffle. In this position, the bit’s joint is adjacent to the horse’s palate and the mouthpiece has relatively little mobility, so it’s difficult for the horse to move the bit to relieve pressure on sensitive structures.

3. KK Ultra:

The loose rings on this snaffle allow the mouthpiece to rotate downward. Because the central link is oriented at an angle to the mouthpiece, the downward rotation of the mouthpiece brings the smooth surface of the central link adjacent to the palate. The central link was located farther from the palate than the joint of a single-jointed bit. Rein tension moved the entire mouthpiece of the KK Ultra away from the palate by compressing the tongue, and the central link rolled upward over the surface of the tongue as tension increased. The relatively large separation between the central link of Lateral-view radiograph of a horse’s head with the KK Ultra in place. Labels P (palate), T (tongue), R (ring), M (mouthpiece) and L (central link). Note the smooth surface of the central link facing the palate.this bit and the palate, combined with the smoothness of the link surface, may explain why many horses appear to be comfortable and perform well in this bit.

4. Myler Comfort Snaffle:

A wide range of Myler bits is available. One of the bits evaluated in this study was the Myler Comfort Snaffle. As shown in as shown at right, the two arms of the mouthpiece meet within a central barrel that allows a swiveling motion but does not permit any nutcracker action. Radiographs showed that the mouthpiece was positioned quite high on the horse’s tongue and tended to press deeply into the tongue, indicating that the tongue muscles were relaxed. The position and angle of the Myler mouthpiece did not change when tension was applied to the reins, but the bit moved away from the palate by further indenting the tongue.

Some horses that resist the action of conventional bits perform well in a Myler bit. This increased acceptance may be related to the smoothness of the surface barrel, the higher position of the mouthpiece on the tongue, the fact that the angle of the mouthpiece does not change when tension is applied to the reins, or all three. Another possibility is that the relatively rigid Myler mouthpiece allows the horse to push against it with his tongue to control pressure on the bars.

The results of our study indicated that the size and shape of the oral cavity vary among horses, and that these differences are likely to affect each horse’s comfort levels with different types of bits. Stay tuned for Part 2, in which we take a close look at how bits move within the horse’s oral cavity.

Hilary Clayton, BVMS, PhD, Diplomate ACVSMR, MRCVS, is a world renowned expert on equine biomechanics and conditioning. Since 1997, she has held the Mary Anne McPhail Dressage Chair in Equine Sports Medicine at Michigan State University’s College of Veterinary Medicine, East Lansing. The position focuses on dressage- and sport horse-focused research. This research was made possible by a grant from the U.S. Eventing Association. The research was performed by Dr. Jane Manfredi when she worked in the McPhail Center through the Merck-Merial Veterinary Scholars Program. Reprinted with permission from the USDF Connection.


Dr. Hilary M. Clayton is a veterinarian, researcher and horsewoman. For the past 40 years she has performed innovative research in the areas of locomotor biomechanics, lameness, physical therapy and rehabilitation, conditioning programs for equine athletes, and the interaction between rider, tack and horse. She has published seven books and over 200 scientific articles on these topics. Since retiring from academia in 2014, she has applied the results of her scientific research to the development of practical techniques to help riders, trainers and veterinarians. She continues to perform collaborative research with colleagues in universities around the world. Dr. Clayton is a lifelong rider and has competed in many equestrian sports, most recently focusing on dressage, in which she trains through the Grand Prix level and has earned US Dressage Federation bronze, silver and gold medals.