Alois Podhajsky's Full Description of The Seat, from p 211 of :
The Complete Training Of Horse And Rider

While the horse is standing still, the trainer explains the correct position to the rider. This begins with the foundation, namely, the seat. This should be pushed well forward into the centre of the saddle. Both seat bones should rest firmly in the saddle so that the coccyx points to the centre line of the saddle. The seat should be open and not pinched together in order to allow the rider to sit as deep as possible in the saddle. Both seat bones resting in the saddle together with the coccyx, which does not touch the saddle, form the "triangle of the seat" mentioned in many old books about riding”.

Both hips, which decide the position of the upper part of the body, must be vertical to the saddle and the triangle of the seat. If the hip comes behind this vertical line, the rider will sit as if he were in a chair, the back would be rounded, the knees would come off the saddle and the legs would slide too much forward. This seat would be as much a fault as sitting on the fork, which comes about when the hip comes in front of the vertical line with the consequence that the legs and knees would be taken too far back. The effect of this seat is even more harmful than the "chair" seat because the rider sits more on the thigh than on his backside. The upper body would become unsteady and the rider would easily lose his balance, besides which any pushing aid of the weight would be impossible”.

The rider's back must be upright with the small of the back braced. The spine must not be hollow and the back must remain supple and flexible. This is necessary to enable the rider to follow all the movements of his horse as if he were a part of his own body. The back must remain firm and upright to allow the rider to use the small of his back as an aid; otherwise he would not be able to prevent the horse from pulling him out of the saddle when lying on the reins.

Here we have another example of how much contrasts combine to make harmony. The rider should sit upright but not stiffly, and he should be completely relaxed without slouching. This harmony can only be the result of a long and systematic training of the human body of which ballet dancers give an excellent example.

If the knees are raised and too far forward they will provoke the "chair" seat. If they come too far back, the leg will nearly be vertical to the ground and throw the rider onto his fork. The knees must lie flat on the saddle and must never move from it. A gap should not be seen between the knee and the saddle.

The lower legs should form a wide angle with the thighs and lie close to the horse's body, hanging down by their own weight without tension. They should be on the girth. The foot, parallel to the horse's side, is the prolongation of the leg turned inwards throughout its whole length. The heel should be the lowest point of the foot.

A vertical line drawn from the shoulders of the rider to the ground should touch his heel and a vertical line from the knee should touch his toes.

In the correct seat, the seat and thighs down to the knees lie close to the horse's body. The upper part from the hips upwards and the legs from the knees downwards are moveable. Their movements must co-ordinate with the movements of the horse. The rider must not fall back as the horse starts to move, or fall forward when he reduces the speed.