Reid's base line

Reid's base line is used for an unambiguous definition of the orientation of the human skull in conventional radiography, computer tomography (CT), and magnetic resonance imaging (MRI) studies.

In 1962, the World Federation of Radiology defined it as the line between the infraorbital margin and the upper margin of the external auditory meatus.

With the head upright, it is typically tilted about 7 degrees nose up with respect to the horizontal plane.