Once you have determined that your horse may need a new bit, see Introduce a New Bit

Fitting a bit starts with understanding the anatomy of a horse’s mouth. For more details see: Horse Teeth. In general terms, his lower jaw is smaller than his upper jaw. His bars are without muscle and full of sensitive nerves that lie very close to the surface as is the palate or roof of the mouth. The distance or height of the palate is different in different horses. Some have shallow mouths with a low palate. Others have a much more concave mouth with a high palate. The tongue is quite sensitive and may be fatter or slimmer in different horses.

A horse breathes only through his nose. The tracheal passage remains open at all times unless the horse is swallowing. When the tongue moves back to swallow (or avoid the bit?), the root of the tongue partially (or completely) obstructs the airways by blocking the trachea and opening the oesophagus to allow food into the stomach. The back of the tongue is attached to the hyoid bone which is attached to his jaw bones and is the origination of the two most important neck muscles: one to the sternum and one to the inside shoulder. This relationship is the basis for the theory that a “stiff” tongue produces a “stiff” neck: a relaxed tongue encourages a relaxed horse. His tongue and his bit are intricately connected to his neck and shoulders. Any discomfort there will manifest itself in ill-behavior in an effort to avoid the tongue/breathing problems.

His lips are also sensitive, particularly in the corners of the mouth

When a horse feels pain, it triggers his “fight” or “flight” response. A horse whose bit is ill-fitted or in the hands of a poor rider can experience pain. His natural reaction will be to “run away” from the pain (in which case you will need more than a bit) or to “fight” the bit. A horse that is “fighting” his bit becomes very tight in the neck, shoulders, top line. Over a long period of time, fighting a bit can cause anatomical and muscular changes to a horse much to the detriment of his ability to maintain his natural balance and harmony of muscles.

Any bit that interferes with breathing or that slides back and forth at the lip corners (as in a too wide bit) or hits sensitive palate areas (as in a high-port bit) is mal-fitted and can cause serious and long term problems with a horse’s attitude and way of going.

How to Fit a Horse Bit

A bit should not cause any significant pain, bruising or cuts in a horse’s mouth.

To achieve the most effective results from bitting a horse, make sure you have measured your horse’s mouth properly – width and height. Also have your horse’s teeth checked periodically. If he shows any signs of mouth discomfort (see Horse Teeth), have your vet give him an oral exam right away. (see Bars)

How to Measure for a Horse Bit

Generally speaking, a bit should extend about 1/4″ -3/8″ beyond the horse’s lips on either side of the face. A bit that is too wide can slip and bang. One that is too narrow can pinch when worked and will exert unrelenting pressure on his face.

The average horse takes a 5″ wide bit. Miniatures might take a bit as small as 3″ and warmbloods and drafts go up to a 5-1/2″-6″ wide mouth. The type of bit may influence how wide it should be. ie: an O-ring snaffle that moves back and forth as well as around the ring should be fitted a little wider than a D-ring snaffle with a stable end to prevent pinching. (If you are using a bit that might pinch your horse’s lips, you will want to use bit guards. If that is the case, add 1/2″ to the width of the mouth as you measure to leave room for the guards.)

If you find it difficult to measure, use a dowel, place it in your horse’s mouth where you want the bit to sit, and mark either side with a marker. You can also use a sewing tape measure (the soft kind, not a hardware tape measure) the same way.

If you already have a bit that fits, measure it to select a new one.

A colt just starting to wear his bit should be fitted with no wrinkles at the corner of his mouth. It is best if he learns to “carry” the bit first instead of having the headstall hold the bit. Later, when he is comfortable, one wrinkle at the corner of his mouth is fine. Some trainers will even use a two-wrinkle rule. The most important thing is to have the bit fit comfortably in his mouth. The O-ring snaffle on this colt could cause pinching at the corner of the mouth when rein pressure is applied, so be aware of that possibility. A bit guard might be in order for such a bit.

(Also be aware that colts – and some adult horses -still have wolf teeth, which can cause bit discomfort.)(see Horse Teeth)

It is very important to keep your horse’s mouth in top shape. His teeth need to be floated regularly to take the sharp edges off the molars. Hooks on the back teeth can prevent the lower jaw from sliding forward, which will inhibit his ability to relax and yield to the bit.

Some performance horses have their teeth floated specifically for bit comfort and fit. By filing the front four cheek teeth to round them back, you can make more room for the bit and make the horse more responsive. (“bit seat”) (see Horse Teeth)

Signs of Mouth Discomfort

courtesy of Dr Dean Scoggins University of Illinois College of Veterinary Medicine

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• Reluctance to be bridled or unbridled
• Head Tossing
• Gapping (opening the mouth)
• Lolling the Tongue
• Chewing the Bit
• Stiffness in the jaw or neck
• Leaning on the Bridle
• Excessive tail switching
• Tilting the Head
• Dribbling Grain
• Chewing Abnormally
• Reluctance to eat
• Blood-tinged, foul-smelling drool

Once you have determined that your horse may need a new bit, see Introduce a New Bit

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