This photo shows proper adjustment of a curb chain on a kimberwick horse bit. The curb chain attaches to hooks called curb hooks on either side of the bit. The curb chain must then be adjusted to an effective length. A curb chain that is too long will have no effect, rendering the bit significantly less effective. A curb chain which is too short will engage with the slightest contact on the rein, which can be painful and lead to head tossing or even a horse rearing or buckle.
The general rule of thumb is that the curb chain should be adjusted so two fingers can fit between the chain and the horse's lower jaw comfortably, but due to the wide variance in shank lengths, it's better to use the two fingers rule only as a guideline, and instead adjust the curb chain so it tightens when the shank (in this case, the straight piece at the front of a kimberwick)is pulled by the reins to a 45 degree angle.