There is no one gullet channel width that is appropriate for every horse.
There is no such thing as “one size fits all” where the gullet channel of your horse’s saddle is concerned. Instead, the width of each horse’s spine will determine how wide his saddle’s gullet channel must be.
To calculate how wide your horse’s spine is, do the following. Stand on your horse’s left side and place your hands on his spine in the area where his saddle will sit. Then, with the tips of your fingers, gently palpate downward towards the ground. You will first feel bone (the transverse processes of the lumbar vertebrae), then a slight rigidity (the supraspinal ligament), and finally, an area where there is a bit more give. This is his back or longissimus dorci muscle. Mark the start of this muscle and then do the same thing on your horse’s right side. Next, take your right hand and make a bridge over your horse’s back from mark to mark. Put your left hand inside that “bridge.” The number of fingers you can get inside your bridged hand will determine how wide the gullet channel of this horse’s saddle must be.
It is very important that the width of the gullet channel be the same throughout the entire length of the saddle. Too often we see saddles with gullet chanels that are the appropriate width at the front, but then progressively narrow towards the back. The result is a saddle that has a 4-5 finger gullet channel width under the pommel, but only 2-3 fingers at the cantle. If you consider the anatomical structure of the horse’s back, this makes no sense. The horse’s spine and surrounding ligaments do not get narrower over the length of his saddle-support area. As a result, in order to ensure adequate spinal clearance, neither should the gullet channel of his saddle.
It is only infrequently that we find a saddle that is too wide through the gullet channel for a particular horse. But such a saddle will have inadequate weight-bearing surface, may start to strip muscle away from the top of the ribs, and the back of the tree may actually rest on the spine.
A much more common problem is a saddle with too narrow of a gullet channel. This saddle will sit on the horse’s spine and/or ligaments. This is especially noticeable when the horse goes around a corner: if the horse is tracking to the left, you will see the saddle shift to the right, so that the left-side panel rests on the horse’s spine/ligaments. This is something we must avoid at all costs. In the short-term, a saddle that sits on the horse’s spine/ligaments will cause him to tighten his back muscles and hollow his back, producing exactly the opposite of the nice rounded back that we want to see, particularly in dressage. In the long-term, a saddle with too narrow of a gullet channel will cause permanent, irreversible, and often career-ending injury or damage to the horse’s back. The most severe forms of such damage are spinal stenosis (compression and narrowing of the spinal canal) and spondylosis (degeneration of the vertebrae).