The girth should be positioned a hand’s width away from the horse’s elbows, so as to avoid injury;
Shoulder movement must not be inhibited. The hollow behind the shoulder should prevent the saddle from moving up towards the withers.
There must be no pressure on the loin.
That is the whole idea behind the design of the Gaston Mercier tree, ensuring that the rider is in the right place over the horse’s 13th dorsal vertebrae.
This is the only position where the rider does not cause discomfort to the horse.