You know it's important for your horse's saddle and bridle to fit correctly. But don't forget to make sure his bit fits, too. A bit that's too small can pinch the corners of the mouth, while a bit that's too big can move around too much and clunk against his teeth. Rubbed patches or thickened skin at the corners of the mouth are signs of bad bit fit, but it's better not to wait to see physical evidence that a bit doesn't fit well.
To test whether a bit is the proper size for your horse, you need only an ordinary piece of twine and a ruler or tape measure:
Guide the twine into the horse's mouth and back toward the corners so that it is positioned approximately where the bit would lie.
Pull it taut and use your fingers to grasp it at each corner of the lips.
Remove the twine and use your ruler to measure the length between your fingers.
The general rule is that a properly fitted bit measures a quarter-inch longer than the width of the horse's mouth. You can adjust the cheek pieces of the bridle to make sure that the bit rests properly in the bars of the mouth, creating one or two wrinkles at the corners.
In addition to width, the overall shape of a horse's mouth, including the hard palate and the size of his tongue, can influence the size of bit he needs. This means that a thicker bit is not always milder. And, as a horse ages, the contours of his mouth may change, so the bit you use now may not be comfortable for him in 10 years. It's easy to start second-guessing your current bit, but don't worry: If your horse is happy and responsive, chances are it fits just fine. If you're concerned, you can always ask your veterinarian to evaluate it.