This position may be modified by the instructor to suit varying conditions and unusual conformations. When not at attention, the head and eyes are directed so as best to favor alertness and observation. In other respects the position should be practically unchanged. =199.= The body and lower legs are movable and should be under the control of the trooper, either acting intermittently as aids for guiding the horse or as a means of binding the rider to the horse while following his movements. The thighs, on the other hand, should remain fixed immovably to the saddle, except while posting at the trot. This fixity should be obtained not by the pressure of the knees but by the clinging of the buttocks, which is secured by the suppleness of the loins and the relaxation of the thighs. It is acquired very rapidly by daily "_rotation of the thighs_," which gradually presses the large thigh muscles to the rear and permits the femur to rest solidly against the saddle. The trooper should sit with his buttocks well under the upper part of his body and especially avoid bowing the back by thrusting the buttocks to the rear and the lower part of the spine to the front. Sitting well forward in the middle of the saddle will tend to assist the trooper in avoiding the defect just referred to. If the buttocks are thrust back too much the trooper can not conform to the movements of the horse and carries forward the upper part of his body. If the thigh is too nearly horizontal, the trooper is doubled, up and his power of action diminished; if the thigh is too nearly vertical, the trooper is on the crotch and lacks ease. To sum up: The trooper should take a relaxed sitting position, squarely on his buttocks, with the thighs inclined downward. The various defects of position are overcome by suitable suppling exercises (pars. 209-220). =200.= _The aids._--The legs, the reins, and the weight are the means of controlling the horse in riding. They are called the aids. =201.= =The legs=: The legs serve to urge the horse forward, to increase his pace or gait, and to engage the hind quarters or move them laterally. The legs act by the pressure of the calves. If pressure alone is insufficient the trooper increases the action by blows with his calves. It is essential to obtain from the horse perfect obedience to the action of the legs. He should respond to the simultaneous and equal action of both legs by engaging his hindquarters and moving forward; to the predominant action of one leg by moving his haunches to the opposite side. =202.= =The reins=: The reins serve to prepare the horse to move, to decrease or increase his pace, to change the gait, or to change direction. =Contact= is a light bearing of the mouth of the horse on the hand of the rider. It should be constantly maintained.