Tilt the head back so that the occlusal plane of the mandibular teeth is at right angles to the horizontal plane with the median plane (sagittal plane) of the face in a vertical position. Place the film packet in the mouth with its short axis in line with the median plane and its posterior border in contact with the mandibular rami. Adjust the tube to direct the central ray along the median plane so as to be perpendicular to the film packet. The ray should pass through the floor of the mouth and the body of the mandible to the center of the film (see figure 4-33).