The measurements of cephalograms play a major role in orthodontic research and treatment planning. It is important to understand that cephalometric measurement, like all measurements, involves error. Three general classes of error occur in the measurement of cranial dimensions. These are, errors of projection, errors of landmark identification and mechanical errors during tracing. An other cephalometric error is the incorrect measurement of the angular and linear measurements of these structures. The purpose of this study is to determine a method to minimise inter or intrainvestigator errors by using geometrical structures and the theorems related to them. For this purpose, on 200 cephalograms traced by 5 different investigator, the sum of the interior angles on the Tweed's triangle was calculated and the mean of the absolute values of errors was recorded. Related to the equation between the sum of the posterior angles in Bjork's polygon and SnGoMe angle (SUM=360°+SnGoMe), the mean of the absolute values of errors was also recorded. The significance of the difference of the mean values for both measurements was evaluated by “student t” test and the difference was found significant at the level of p<0.01. Since in cephalometric studies lots of geometric polygons exists, as conclusion we emphasised that geometric theorems must always taken into consideration to obtain consistent analysis.

Keywords: Cephalometrics, measurement error, Tweed’s triangle, Björk’s polygon