When it comes to bit fit, width and thickness are the two measurements most riders focus on — and with good reason. Getting both right is fundamental to the comfort and stability of the bit in the mouth. Getting either wrong creates problems that reach far beyond the mouth, affecting the horse's willingness to accept contact, their straightness, and the clarity of communication between horse and rider.

Yet both are frequently misunderstood. The common rules of thumb — "bits should be X millimetres wide" or "thicker is always milder" — are oversimplifications that can lead to poor choices even when riders are genuinely trying to do the right thing for their horses.

Bit width — why it matters and how to get it right

What happens when the bit is too narrow

A bit that is too narrow pinches the lips and corners of the mouth between the rings or cheekpieces and the teeth, creating constant pressure and rubbing that can quickly become acutely painful.

You will often see horses with a too-narrow bit displaying red, sore lips or small wounds at the corners of the mouth. These injuries are sometimes misattributed to the horse being strong or resistant — when in reality the horse is simply reacting to pain. Over time, repeated rubbing and pressure create scar tissue that complicates future fitting and causes ongoing sensitivity that may persist long after the original cause has been addressed.

A too-narrow bit also restricts how the bit moves within the mouth. Loose ring snaffles in particular rely on a small degree of free movement to function correctly — signalling softly and clearly as the ring rotates through the mouthpiece. A loose ring that is too tight cannot move as designed, and the result is a bit that pinches simultaneously and fails to communicate clearly. It is, in effect, the worst of both worlds.

What happens when the bit is too wide

A bit that is too wide slides from side to side in the mouth with every movement of the reins. Contact on one rein disappears briefly before reappearing on the other side — an inconsistency that creates confusion, anxiety, and a horse that learns to brace through the jaw in an attempt to stabilise the bit themselves.

When the bit slides laterally, the mouthpiece can also extend too far through the rings on one side when the opposite rein is taken up, potentially making contact with the teeth and creating a sharp, unexpected pressure the horse will naturally evade.

Riders often find themselves using more rein contact than is ideal simply to try to stabilise an unstable bit — inadvertently creating further tension rather than resolving it. A horse cannot learn to trust a contact that is unpredictable.

What correct width looks like

For most snaffle designs, correct width allows approximately 5mm of clearance on each side of the mouth when the bit is held centrally — enough to prevent pinching whilst maintaining the stability that allows the bit to function correctly.

This is a guideline, not an absolute rule, and the appropriate clearance varies depending on the specific design.

Loose ring snaffles require a small amount of additional room to allow the rings to move freely through the mouthpiece without pinching the lips between the ring and the mouthpiece — a problem known as ring pinch. This is a common and frequently overlooked source of discomfort.

Fixed-cheek designs — eggbutt snaffles, full cheeks, D-rings — do not carry this risk because the cheekpieces cannot rotate through the mouthpiece. They can be fitted slightly more snugly without the same pinching concern.

Understanding the design characteristics of the specific bit you are using is an essential part of selecting the correct width. For a step-by-step guide to measuring your horse's mouth accurately, see our how to measure your horse's mouth for a bit guide.

Mouthpiece thickness — the myth of "thicker equals milder"

The relationship between mouthpiece thickness and severity is one of the most discussed — and most misunderstood — topics in bitting.

The general principle

Thinner mouthpieces (typically 10–14mm diameter) concentrate pressure over a smaller surface area. The narrower the mouthpiece, the more focused the pressure at the point of contact. This makes thin bits sharper and more direct in their action — not inherently inappropriate, but requiring greater care and lighter hands than thicker designs.

Thicker mouthpieces (16mm and above) distribute pressure over a broader surface area, creating a gentler, more diffuse sensation. For this reason, thicker bits are often recommended for young horses, sensitive or anxious horses, and horses being rehabilitated after a history of bitting problems.

So far, so straightforward. But here is where the common wisdom breaks down.

The critical caveat — thickness must match mouth conformation

A thicker bit is not always milder. For a horse with a small mouth, a low palate, or a large tongue relative to the space available, a thick bit can cause considerable discomfort — in some cases more than a thinner, well-chosen alternative.

The tongue is a large, muscular organ that fills the majority of the mouth cavity in most horses. In a horse with limited mouth space, adding a thick bit into an already crowded environment creates compression the horse cannot escape. The bit may press against the palate — the roof of the mouth — creating a source of pressure that manifests as:

• Consistently trying to get the tongue over the bit
• Evasion of contact and going behind the bit
• Excessive mouth opening in an attempt to relieve pressure
• Head tossing when contact is taken up

Horses with low palates are particularly vulnerable. The distance between the top of the tongue and the roof of the mouth may be very small, and a bit entirely comfortable in a horse with a normal or high palate can make consistent palatal contact in a horse with a low one. This can only be identified through a thorough assessment of mouth conformation — it cannot be determined from external measurements alone.

For horses with low palates, small mouths, or limited tongue space, a thinner mouthpiece may actually be more comfortable than a thicker one — providing more room for the tongue and reducing the risk of palatal pressure. This apparent contradiction is one of the clearest demonstrations of why individual assessment matters so much in bit fitting, and why applying blanket rules about thickness leads to poor outcomes.

Mouthpiece shape and design

Beyond width and thickness, mouthpiece shape profoundly influences where pressure falls and how the bit acts in the mouth.

Straight bar mouthpieces act primarily on the tongue, bars, and corners of the mouth simultaneously. They are stable in the mouth and suit horses that accept tongue pressure well, but offer limited tongue relief.

Single-jointed mouthpieces create a nutcracker-like action when rein pressure is applied — the joint rises in the centre while the two halves rotate downward and inward. This action applies pressure in different proportions to a straight mouthpiece, and some horses find the upward movement of the joint uncomfortable, particularly if they have a low palate. For more on this, see our guide on understanding tongue pressure in horses.

Double-jointed mouthpieces — those with a central lozenge, French link, or Baucher plate — significantly reduce the bar squeeze action by introducing a second point of articulation. The joint cannot rise sharply in the centre of the mouth, making the action gentler and more even for many horses. They are often a useful first step when a horse has shown tension or evasion with a single-jointed snaffle without any other obvious cause.

Ported mouthpieces create a channel for the tongue to sit in, reducing direct tongue pressure and directing more contact onto the bars. The height of the port determines the degree of tongue relief — but port height must always be considered alongside palate height. A port that provides welcome tongue relief in one horse can create uncomfortable palatal contact in another. See our complete guide to bit mouthpieces for a full breakdown of design options.

Bit material and its influence on acceptance

The material from which the mouthpiece is made can also influence the horse's acceptance of the bit — and it is worth considering alongside width, thickness, and design.

Stainless steel is the most widely used material and is entirely suitable for most horses. It is durable, easy to clean, and inoffensive to most horses.

Sweet iron oxidises slightly in the mouth, creating a mild, slightly sweet taste that many horses find more palatable. It tends to encourage gentle mouthing and salivation — both signs of relaxation and acceptance.

Copper and copper alloys (such as aurigan) work on a similar principle — encouraging salivation and a relaxed jaw through taste. A dry mouth is often a sign of tension, and a bit material that encourages gentle salivation can make a meaningful difference to a horse's overall acceptance of contact.

Rubber and synthetic materials create a softer feel against the bars and tongue and can be useful for very sensitive horses or those with a history of bar damage. They tend to be bulkier, so mouth size and tongue space must be considered.

The choice of material is rarely the primary issue in a bitting problem, but it can make a genuine difference at the margins — particularly for horses that are slow to salivate, tense in the jaw, or resistant to accepting the bit into the mouth. See our complete guide to bit mouthpiece materials for a full breakdown of mouthpiece material options.

Key takeaways

✔ Width: 5mm clearance each side is the general guideline, but loose ring snaffles need slightly more room to prevent ring pinch

✔ Too narrow: pinches lips and corners, restricts bit movement, causes pain and scar tissue

✔ Too wide: creates instability, inconsistent contact, and a horse that braces to stabilise the bit themselves

✔ Thicker is not always milder — for horses with low palates, small mouths, or large tongues, a thicker bit can cause more discomfort than a thinner alternative

✔ Mouthpiece shape determines where pressure falls — single-jointed, double-jointed, straight bar, and ported designs all act differently and suit different horses

✔ Bit material influences acceptance — sweet iron and copper encourage salivation and a relaxed jaw

✔ All of these factors interact with each other and with the individual horse's mouth conformation — which is why individual professional assessment is the only reliable way to get it right

Find the right bit for your horse

At The Fitted Horse we assess your horse's complete mouth conformation — palate height, tongue size, bar sensitivity, jaw width, and dental health — before making any bit recommendation. Width and thickness are part of the picture, but they are only the beginning.

Book a bridle and bit fitting consultation →

Further reading: Why a correctly fitted bit matters | Understanding tongue pressure in horses | The complete guide to bit mouthpieces | Choosing the right bit for your horse's mouth conformation | How to measure your horse's mouth for a bit