For a bit to work correctly, it needs to be sitting in the mouth correctly, and that means it must fit. A bit that is too small may pinch or make it hard for the horse to close its mouth properly. A bit that is too large will slide back and forth in the horse’s mouth, perhaps even knocking on the horse’s teeth. In either case, your rein aids will not be accurate, and your horse won’t be getting the right messages about stopping and turning. Traditionally, horsemen have used the ‘one wrinkle’ rule to determine if the bit is sitting in the right place in the mouth and fits correctly. One wrinkle at the corner of of the horse's mouth meant the bit was sitting in the right place. This isn’t always accurate and doesn't necessarily mean the bit fits or is adjusted properly. The bit should fit comfortably across the bars (the toothless gap between the incisors and molars) of the horse’s jaw, and that may mean there isn't just one wrinkle or any wrinkle at all.
If you fit a jointed bit, like a D-ring or loose ring snaffle, there may be no wrinkle on the lips at all. That doesn't mean the bit doesn't fit. The way the bit looks on the outside may not have anything to do with what is happening the horse's mouth. Take a look inside the horse’s mouth by lifting the lips and notice where the bit is sitting on the bars. It should not be sitting so high or low as to hit the teeth on either side of the bar. Pay attention to the horse’s reaction too. Your horse might tell you by chewing and mouthing the bit, or other behavior, that it’s not comfortable. If the bit isn't positioned in the mouth evenly, the horse could start carrying its head to one side or toss its head. Adjust the bridle so that the bit sits balanced on both sides and sits comfortably in the center of the bars.
Fitting the bit might not just be a matter of positioning but of the size and shape of the bit itself. Horses have differently shaped mouths, meaning different bits may be more comfortable for some than others. You might have measured your horse’s mouth and decided to buy a 5-inch bit. However, the length of the mouthpiece is not the only thing to consider. Horse’s mouths come in different sizes and shapes, just like two people may have the same sized feet but won't necessarily be able to wear the exact same type of shoe.
For example, you might think of a very thick eggbutt snaffle as a very soft comfortable bit, but for a horse with a fleshy tongue and low palate, it may be an uncomfortable mouthful. The bit may fit width-wise, but not in thickness. So if you’re having trouble getting your horse to carry a bit quietly, consider trying another type of mouthpiece. Some horses like jointed mouthpieces, or seem more comfortable in different types of joints, such as a French link rather than a loose ring snaffle. Some horses may be happier with a bit with a port that gives some room for their tongue. The width, shape, and thickness of the mouthpiece are factors in fitting the bit correctly.
It can take a bit of trial and error to find the correct position in the horse's mouth for the bit. Try adjusting the bit according to the where the bit is sitting in the horse's mouth, and try different types of mouthpieces as well. Your horse's bit may be the correct size, but if it isn't sitting in the correct position, it won't be comfortable or effective.