Selecting the proper seat size for the rider can sometimes be a frustrating experience. All saddles have a listed seat size so you would think it would be a fairly straight forward process. If you’ve ridden a 15 inch saddle before all you need to do is make sure your new saddle is the same size. Sometimes this works fine and other times it’s a disaster.
Some saddle makers measure from the cantle to the top of the hole in the pommel or fork. Others measure from the cantle to the base of the horn. This will give you two different measurements on the same tree.
Another area to consider is the slope of the cantle. A more pitched back cantle will measure longer than a straighter cantle but can have the exact same size opening from the base of the pommel or swell to the base of the cantle where the thigh passes through.
As the cantle height increases, it also goes farther back, away from the fork, which increases the measurement of the seat without changing the thigh opening in the saddle for the rider at all.
One other thing to keep in mind is personal preference of the rider. Some people like a more closed in seat and others prefer a more open one. We come across a fair amount of people who learned to ride in a saddle that wasn’t exactly the right size for them. They get used to it and that becomes what feels right for them.
When considering seat size for the rider it is more important to make sure you have enough room between the base of the fork and the base of the cantle for your thigh to fit comfortably and not get hung up on an arbitrary seat measurement.