Two proportional methods are introduced to determine the normal position of the floor of the 4th ventricle expressed by two preventricular ratios: (see article); where ds = dorsum sellae, Ts = tuberculum sellae and Tw = Twining's line. T4 is the intersection between Tw and the floor of the 4th ventricle. The methods give information of the position of the 4th ventricle, the diameter of pons and the a-p diameter of the pituitary fossa.