A line drawn from the basion (the midpoint of the anterior margin of the foramen magnum) to the inferior posterior C2.  A perpendicular line is then drawn from the center of this line to the dura of the brainstem.  A Grabb-Oakes measurement greater than 9 mm denotes a form of basilar invagnation.  This is a very helpful measurement for determining how much a retroflexed odontoid is compressing the brainstem.