9. Check the seat length.
The weight-bearing surface of a horse’s back is the area that is supported by the ribs. This area is known as the thoracic region. It runs from about the point of his shoulder to the middle of his back. The 18th vertebra represents the end of the thoracic, or weight-bearing, area and is associated with the last rib.
The lumbar region of his back has no ribs and therefore no support structure; it should not bear weight. This area runs from about the middle of his back to the point of his croup.
Feel your horse’s rib cage to locate his last rib. Follow that last rib up to his spine, and you’ll see the approximate location of the 18th vertebra and end of the thoracic area. Ideally, your saddle should not extend past this point. If it does and you have no choice but to ride in a saddle that is slightly too long, be sure to check your horse continually for signs of soreness. Some saddles feature more compact designs than others, and one of those may be more appropriate for your horse.
10. Girth the saddle, mount and recheck the fit.
When you’ve finished all the steps to check the saddle fit, put on a girth but consider skipping the use of a saddle pad. A properly fitted girth sits approximately 5 inches behind the horse’s elbow. While sitting in the saddle, check the wither clearance and gullet clearance again.
The pommel should still clear the withers by two to three fingers. Your helper—being careful not to be kicked—should be able to see daylight running the length of the saddle when looking at the saddle from the rear view. This is easiest to see without the use of the saddle pad.
Notice the feel of the saddle. It should feel stable and level under you, and you should feel balanced—not like you’re leaning backward or forward or struggling to sit up straight.
Again, assess how your horse is feeling. Is he able to move out freely? Is he relaxed? Or are his ears pinned and tail swishing?