Why the mouth deserves your full attention

A bit is not a brake. It is a conversation. Everything you feel through the reins travels down to a few square centimetres of tissue on your horse's lower jaw, and everything your horse feels about your hands arrives through the same route. When that conversation is comfortable, you get softness, rhythm and a horse who stretches into the contact. When it is not, you get tension that shows up as head tossing, hollowing, rushing, napping or a tongue that appears over the bit in the warm-up.

It is worth knowing what is actually in there. Across the gap behind the incisors sit the bars — the smooth, rounded edges of the lower jaw, covered in thin gum tissue and sitting very close to the bone. Below and around them lies the tongue, which in most horses fills a surprising amount of the mouth. Above sits the palate. A bit that suits one horse can be genuinely painful for another with a thicker tongue, a narrower jaw or a lower palate, which is why copying what the horse next door wears rarely works.

Measuring mouth shape before you buy anything

Start with width. A bit should sit with roughly half a centimetre of clearance on each side of the lips — about one centimetre in total. Too narrow and it pinches the corners; too wide and it slides from side to side, catching the lips and giving you a vague, wandering contact. For a loose-ring bit, add a little extra to allow the rings to move without dragging.

You can measure with a proper bit measure, or simply slide a clean piece of dowel or a soft tape across the mouth where the bit will rest, then mark either side of the lips. Do this with the horse relaxed and not eating. Then assess:

• Lips: thin, tucked lips need a smaller diameter mouthpiece; thick, fleshy lips tolerate a little more.
• Jaw width: a narrow jaw means less room between the bars, so thickness matters even more.
• Tongue size and height: a big, thick tongue sitting high in the mouth leaves very little space for a bit above it.
• Palate height: a low palate is easily bumped by a joint or a thick mouthpiece.

Tongue clearance is where most problems hide

The single most common cause of bit discomfort I see is crowding. The tongue needs somewhere to go. If the mouthpiece is thick and the tongue is generous, the bit presses the tongue down against the bars and up against the palate, and the horse has no choice but to resist.

Watch for the quiet signs: a tongue that flickers or comes over the bit, a jaw that sets rather than chews, head tilting, a horse who opens the mouth to evade, or one who leans heavily and feels dull in front. A horse who has learned to brace is not being naughty — he is protecting himself.

Where clearance is tight, a thinner mouthpiece is usually kinder than a padded one. Double-jointed bits with a lozenge or a French link spread pressure more evenly than a single joint, which can create a nutcracker effect on the bars and a sharp bump on the palate. A gently curved or ported mouthpiece can give the tongue somewhere to rest, provided you are honest about how much room you actually have.

Choosing the mouthpiece: material, thickness and joint

Mild metals and synthetic materials all have their place. Copper and sweet iron encourage salivation in many horses, though a few are sensitive to them. Rubber and synthetic mouthpieces suit very thin-skinned horses but are often bulkier, which matters if clearance is already tight. As a working guide, a mouthpiece of around 14 to 16 millimetres is a sensible starting point for most horses, moving thinner only if the mouth demands it.

Be sceptical of anything promising instant submission. The best bit is the one your horse accepts quietly, not the one with the strongest action.

Fitting the bit and testing rein pressure

Height is easier to get right than people think. In a snaffle, aim for one or two soft wrinkles at the corner of the mouth — enough that the bit does not clatter against the teeth, not so high that it drags the lips upwards. Loose-ring bits can sit slightly lower and more freely; a bit that sits too low will knock against the lower incisors.

Then check for pinching. With the bridle done up, look at the corners of the mouth and run a finger gently under the cheek pieces. Nothing should be trapped between the bit ring and the lip. Make sure the bit is centred, that the cheek pieces are even, and that the headpiece is not pressing on the base of the ears.

Finally, test pressure. With a light rein, your horse should respond to the mere weight of the reins rather than a pull. If you have a luggage scale, you can measure how much tension it takes to get a response — many horses work happily well below the level most riders assume. Release immediately when he softens and watch for the stretch: a comfortable horse will often reach down and forward the moment the pressure comes off. A horse who stays tight, or who throws his head up, is telling you something.

When to bring in expert help

Rule out the obvious first. Teeth should be checked every six to twelve months by a qualified equine dental technician or your vet, because sharp enamel points and loose caps cause far more headshaking and evasion than any bit. Then look at the rest of the picture: a bridle that presses behind the ears, a saddle that has changed shape, and unsteady hands will all mimic bit problems.

If you have adjusted width, height and mouthpiece and your horse is still unhappy, it is worth asking a qualified bit fitter or an experienced instructor to watch you ride. Ten minutes of informed eyes on the ground beats months of guesswork, and your horse will thank you for it.