Angle indicators on the lateral cervical X-ray in the neutral position. Angle A: angle between Line 1 and Line 2; Angle B: angle between Line 1 and Line 3; Angle C: angle between Line 3 and Line 4; Angle D: angle between Line 5 and Line 6; Angle E: angle between Line 7 and Line 8; Angle F: angle between Line 8 and Line 9. (Line 1: a line parallel to hard palate; Line 2: a line passing through the anterior point of the bodies of atlas and axis; Line 3: a line passing through the airway midpoint crossing the cricoid cartilage; Line 4: a line parallel to epiglottis; Line 5: a line along the occlusal surface of maxillary teeth; Line 6: a line passing through anterior-inferior border of the sixth cervical vertebra and the most anterior aspect of the first cervical vertebra; Line 7: a line passing through the posterior-superior point of hard palate and the lowest point of the occipital bone; Line 8: a line passing through the anterior-inferior point and the posterior-inferior point of the second cervical vertebral body; Line 9: a line passing through the anterior-inferior point and the posterior-inferior point of the sixth vertebral body)