We have discussed saddle fitting on several threads to my knowledge.
The difference between a Western saddle and an ENglish saddle is that the English saddle has a much smaller area through which to transfer the weight of the rider to the back of the horse. The pounds per sq inch factor is much higher and a badly fitting saddle will press down on the horse's back and cause discomfort. Some pads do give some relief but essentially the saddle was designed to sit on the horse's back without any interface.
Look up the web sites of the various manufacturers - they all mostly give tips as how to choose a suitable saddle and how to fit it. But nothing beats the "experienced eye" of a saddle fitter.
The saddle must be level, the spine must not be impeded neither must the shoulder and the saddle should be no longer than the length of the weight bearing length of the horse's back. English saddles are divided into patterns ie gp or dressage, length ie 16.5 inch to 18inch and width ie standard then wide to extra wide.
But equally it is very important that the rider feels comfortable - so the rider must sit up on a saddle which already fits the horse and then decide whether or not he/she feels comfortable.
If the saddle does not fit the horse, then lessons to improve the posture or seat of the rider are wasted.
The horse won't get the correct message.
It is equally as important to have a correctly fitting bridle and a bit which suits the horse. A horse strapped up with ill fitting gear is useless to anyone but a novice rider.
who won't know the difference.
If the rider is presented with a tacked up horse by an owner, then all he/she can do is to look to make sure the straps are correctly adjusted. But once the horse belongs to the rider then it becomes the rider's responsibility to check that the tack fits the horse and if you are not sure then to ask someone who is sure.
The horse relies on signals/aids/cues/instructions/ given by the rider to perform - if the saddle, the bit or the bridle doesn't fit, then the horse gets the wrong messages.
For this reason a Western saddle could be seen to be an easier option since these saddles are designed to spread the rider's weight over a larger area. It is also means that the wider saddle will fit a wider range of horses.
But I am told that the modern Western saddle calls for expert fitting as much as an English saddle.
If Grandma has an old ENglish saddle up in the attic, the chances that it will fit a new horse correctly are slim.
Buying a saddle over the internet is fraught with disaster.
I have 4 saddles in my tack room, all belonging to horses that I have owned. None of them fit my present horse - I went out and bought her a new saddle and I drove her in a horse trailer for 3 hours to allow the local expert fitter to choose for her the correct saddle - which I merely had to approve of - not choose myself. But the saddle fitter watched me ride my own horse to make sure the horse was comfortable.
Take care with this matter of tack fitting - learn the significant facts. After the choice of the horse, it is the next most important thing to do.
B G