Fig 2.   A digitized tracing. Maxillon is a point just below (occasionally above) the Key Ridge, midway between the upper and lower border of the palate. Occipitale is the lowest point on the occipital bone. Point Sella is the  origin (X=0.0, Y= 0.0)  of the Cartesian coordinate system used to create the "mathematical model" of the skull. The plane of orientation was given the name "the Krogman-Walker" plane of orientation in honor of the those researchers.