The most misinterpreted concept ever.
Let's have a little chat - you don't know, what you don't know, so allow me to educate you.
One of the most common things I have seen on my fitting journey so far, is people misunderstanding a narrow wither for a narrow tree. This is very minorly the case from what I have seen in my experience so far!
Let's not confuse TREE WIDTH & GULLET ANGLE. These are two separate things.
The TREE WIDTH, needs to allow for clearance of the wither, it is important to ensure this is on top AND THE SIDES. Without giving clearance to the sides of the wither and trapezius muscle, the horse will not be able to bend and will turn stiffly. This is because as it tries to turn and/or bend, a narrow tree width that makes contact with the sides of the wither will pinch. In many cases this may also mean the gullet channel (clearance underneath between the panels) is also too narrow, causing severe pinching of the spine.
As nature recalls, when a stallion mounts a mare, he often bites her right on the trapezius muscle (wither), this causes stability in the front legs, hollowing of the back and rotation of the hips for easy insertion.
Do you want to stabilize your horses front legs? Hollow it's back and tilt it's pelvis all whilst making it move at gait, bend, turn, jump? No way! Avoid pinching the wither!
GULLET ANGLE - in a scenario of having an extremely narrow gullet, all because your horse is "high withered and appears narrow", runs the risk of literally chipping away cartilage and bone of the scapula/shoulder blade.
Why? Because as the shoulder comes back and rotates as the horse moves, each front foot fall means the scapula has rotated back in its sequence, if the GULLET ANGLE is too narrow (OR even too wide), this will hit the shoulder blade. This will result in short choppy strides as the horse tries to avoid that metal reinforced gullet plate, or the horse chips away it's own shoulder as it tries to please you and do as asked (or in some cases, demanded with punishment).
This is only a tiny, tiny taste of many things about saddles and fitting them. A scenario example of the consequences when a narrow saddle is put on a horse because it is "high withered or appears narrow" and does not actually require it.
Below I have attached a few images of my own Thoroughbred for reference. She is 6 years old, a fine, lean athletic type. She has a high-ish wither, but is looong. Many saddles go too low too quick for her and many catch the base of her wither under the twist. She appears to be a rather narrow type (especially standing next to my crossbred ), she is currently ridden in a Medium/Wide - Wide gullet plate with a saddle of great tree width for her type at this time.
Will she require another kind of saddle as she builds muscle and strength? Maybe. But that is a story for another day .
- the blog of a training saddle fitter.