Figure 1-23. - Ala-tragus line parallel with the floor.
Figure 1-24. - Line from the corner of the mouth to the tragus parallel with the floor.
When the tube head has been set for the proper vertical angulation, center the tube head cylinder on the area to be radiographed. The cylinder should almost touch the surface of the patient's skin. Then, position the tube head for correct horizontal angulation.
HORIZONTAL ANGULATION. - This is the side-to-side positioning of the tube head. Position the tube head so the central X-ray beam is directed straight through the embrasures of the teeth being radiographed. If the horizontal angulation is faulty, the central ray will be directed at an angle to the embrasures. This will produce a faulty radiograph, with the images of the teeth overlapping one another. Figure 1-29 illustrates the correct and incorrect cylinder direction. 1-20