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Aim: To evaluate the degree of the reliability of the S–N reference line by using highly stable referencelines (vertical and horizontal cranial axes). Materials and Methods: The sample of the studycomprised of lateral cephalometric radiographs of patients 13–18 years, 12 males and 12 females foreach of the three skeletal relationships. The ANB angles were 0–2, more than 2 and less than zerorespectively. The method was conducted by localization of the anterior superior of anterior wall of sellaturcica point (As), which is stable at age 5–6 years, drawing the Vertical Cranial Axis (VCA) whichpass through the point As and tangent to the upper part of the anterior wall of the sella turcica (at leastfor 3 mm); then, drawing the Horizontal Cranial Axis (HCA), which is perpendicular to the VCA at theAs point. The deflection and the sagittal dimension of the S–N line were measured to evaluate thevariation in the location of the points S and N, which are the determinant of the S–N line. Results:Point N had significantly local variation between gender and among the three skeletal relationships, butthat point S had insignificant local variation between gender and among the three skeletal relationships.Conclusion: The S–N line is not stable due to unstability of the location of point N.

Vertical cranial axis --- Sella turcica --- Frankfort horizontal plane.