Fig. 1.

A. On the axial image, an antero-posterior tangent line "x" is drawn, and line "y" and "z" are drawn perpendicular to line "x" through the center of the mental foramen, then these lines become perpendicular mutually. B. On the coronal image, line "x" is moved to line "x°", the bisecting position of the bucco-lingual dimension of the mandibular body. C. On the sagittal image, line "z" is moved to line "z°", parallel position with the mandibular inferior border of the mandible, then line "y" corresponds with line "y°", perpendicular line "z°" at the same time.