[Where is the right angle?].
Cephalometric radiographs of 49 cases with normal occlusion in the period of permanent dentition (all cases were orthodontically untreated) to study the angle of mandibular incisors obtained by Tweed's and Kubein-Meesenburg's methods. It is known that in both methods the angle should be 90 degrees in cases with normal occlusion. The results obtained were statistically treated and compared. It was found that majority of cases showed value of the angles to be 90 degrees or very close to it obtained by both methods. This finding indicated that two planes were parallel in majority of cases investigated. A change of the Tweed's angle produced change of the Kubein-Meesenburg's angle in such a way that sum of these two angles was 180 degrees. Inclination of the lower incisors showed a considerable variability which indicated an individual sagittal position of these teeth.