When viewed from the front (or back), the skull should be vertical, the atlas (1st vertebra) should be level and the cervical spine (neck) should be vertical - in other words, they should be perpendicular AKA orthogonal.

When assessed on a X-ray, Atlas Orthogonal Doctors of Chiropractic use this criteria to determine the quantity and quality of subluxation. Any departure from orthogonal is less than optimum and should therefore be corrected.