To the uninitiated, and even to more than a few riders, saddle fit is a great unknown. I didn’t understand how a saddle had to fit a horse until I’d been riding for many years and a very kind clinician pointed out to me that while my saddle was a very good one, it wasn’t good for my horse (oops!).
Saddles are not a “one size fits all” deal and there are many parameters to be considered. One of the first is the width of the tree. Depending on the brand you might see saddles marked as Narrow, Medium and Wide (with some offering in-between widths) or in centimeters such as 29 cm or 32 cm. This refers to the distance between the tree points and that distance determines the angle of the tree. A narrow tree (which is generally a 29 cm) will look more A shaped than a wide tree, which will look more like an upside down U.
Much like dress sizes, different saddlers measure in different ways (for example, Stubben saddles typically have longer tree points, so a saddle that measures as a 30 cm may be narrower than another brand that also measures 30 but has shorter points) so while you might think your horse takes a Medium tree in one brand, he might need a Medium Wide or a Medium Narrow in another!
What is important when determining tree width/angle is that the shape of the saddle must conform to the shape of your horse and yet still keep the saddle off of the horse’s spine (and the nerves that run down it). A tree that is too narrow will pinch the horse at the withers and tip the saddle back so that the rider’s weight is too much on the cantle. A saddle that is too wide will sit ON the horse’s withers and can potentially cause nerve damage.
Note: Some people also use “gullet” as an interchangeable term for “tree width”. That’s not really accurate as the gullet (the width of the channel between the panels) does not reflect the angle of the tree, which must conform to the shape of your horse.
Here is a video from Schleese that helps you understand how to evaluate the width of the tree your horse needs.