The position in which a rider sits on a horse. There are numerous types of seats that correspond to different disciplines. The most common and versatile style is called either "basic" or "balanced" seat, which can be adapted easily as a rider progresses to different disciplines.
The basic seat or balanced seat positions the rider squarely in the saddle, with weight distributed evenly on both sitting bones down through both stirrups. Your head is up and your eyes should be looking ahead. Shoulders should be square and relaxed with arms hanging comfortably. You should be able to draw an imaginary line from your elbow through your wrists and reins to the horse's mouth. The height of the hands, therefore, depends on the conformation of horse and rider.
Your upper body is held in a position comparable to proper posture for sitting in a chair, with suppleness and softness in your abdomen that allows for absorption of the horse's motion at the trot and canter. Your hips should be level, with even weight on each buttock. Your thighs should hug your horse with knees pointing straight ahead, not pinching inward or flopping outward. Your calves should lightly contact the horse's sides (exact amount of your leg that contacts horse depends on the size of horse's barrel since smaller horses' barrels tend to slope inward just below the rider's knees).
The lower leg is carried in such a manner that an imaginary line can be drawn perpendicular to the ground from the rider's knee to her toe. The heel is down but not pushed down in a manner that locks the ankle. The ankles should be softly held in place. Rigidity creates a stiff and ineffective leg. The stirrup is usually set so that it reaches to just below the ankle when the feet are disengaged. Variations of leg position are normal because physical differences do not enable all riders to conform to this standard.