This way we get the clearance we need in the pommel without the need to make the arch too narrow. Girthing up and the rider’s weight do not then make the saddle look as if it has been “sucked” down into the horse’s back – the greater part of the rider’s weight remains where we want it from the 10th to the 18th rib.
Horses with high withers, particularly the fit Event horse, often have the added problem of a much wider, well muscled shoulder which can move the saddle across the back as the horse extends its movement and the scapula comes back underneath the forward projection of the flap in a jumping saddle. This torsion behind the wither when the horse is doing fast work and of the saddle moving backwards can cause friction and consequent discomfort against the spine because the well muscled shoulder moves the saddle at every stride.
As the rider adopts the two point seat for fast work or over a fence, the back of the saddle must stay on the horse’s back. When you see the saddle move off the back when the horse is working or over a fence it is usually because there is insufficient support in the front third of the saddle – all the rider’s weight is taken by the stirrup bar which is directly attached to the points of the saddle and the downward pressure exerted on the horses back can increase to approaching 20 times the pressure when the rider is sitting in the seat of the saddle.
This is a "curlier" tree with a higher head and longer points: suitable for horses with a higher wither and more shape to their backs.
This is a "flatter" tree with a lower head and shorter points: suitable for horses with low withers and broad, flat backs.