A patient has been advised a lateral cephalogram before beginning orthodontic treatment. Which of the following could be used to establish the physiologic “true horizontal” orientation of the patient’s head during this radiograph?

In contemporary usage, cephalometric radiographs should be taken in the natural head position (NHP), so that the physiologic true horizontal plane is established. Although NHP is not as precisely reproducible as orienting the head to the Frankfort plane, the potential errors from lower reproducibility are smaller than those from inaccurate head orientation. Ref: Contemporary orthodontics, William Proffit, 4th edition page 208