Proper Bit Adjustment
I'm often asked if there should be one or two wrinkles at the corners of the horse's mouth when the bridle is properly adjusted. There is no correct answer to this question, as the way a bit is adjusted depends on the conformation of the horse's mouth. Some horses have a shallow mouth and lips, and deeper-set bars (see figure A below). By the time the mouthpiece of the bit is properly adjusted on the bars, there may be a wrinkle or two at the corners of the lips. Other horses have a very deep mouth and lips, with bars set more forward (see figure B). These horses may have a minimum contact at the corners of the lips when the bit is properly adjusted. For this reason, the old "two wrinkles at the corners" adage shoud be thrown out of the window.
When adjusting the bit, make sure it is contacting the corners of the lips and not just hanging loose, or you risk it banging around and hitting teeth. Fit it to the corners of the lips, over the bars. Run your index and middle finger along each side of the mouthpiece and work the reins to make sure the bit isn't going to hit any teeth when used. Then touch the horse's tongue to make sure he can't get his tongue over the bit. If everything checks out, you can ride on. However, you may want to adjust the bit up or down, depending on how the horse responds to your cues.