1. The ANB difference is not always an accurate method of establishing the actual amount of apical base divergence. In addition to the geometric shortcomings previously pointed out, the results of this study indicate that the ANB landmarks are subject to growth and treatment mechanics which cause corresponding variations in the measurements used. 2. The AXD method described in this study demonstrates a method of relating apical bases which eliminates two of the variables, nasion and point B. The AXD angle is a more critical evaluation and was found to correspond significantly with a linear measurement of the actual apical base difference. 3. This study and others have demonstrated conclusively that angular measurements cannot compensate for divergence of apical bases resulting from variations in vertical facial height. For this reason, a set of linear measurements was proposed which would offer an accurate method of evaluating the pre- and posttreatment changes taking place. 4. The linear measurements introduced in this study demonstrate an accurate method of assessing apical base relationships. The use of these measurements provides a critical evaluation of treatment and places proper emphasis on favorable growth and treatment mechanics. 5. The study of the pre- and posttreatment linear measurement concerning point A determined that this landmark remained remarkably constant during orthodontic treatment. It would appear from this study that treatment mechanics for Class II cases do not actively relocate point A but actually hold it in position while the middle face is growing forward. 6. The A-D' measurement is concerned with the actual distance, in millimeters, between the apical base landmarks. In the fifty cases studied, the mean pretreatment measurement of A-D' was reduced 15.8 per cent as the Class II malocclusion was corrected. The mean pretreatment ANB underwent a reduction of 43.1 per cent on these same cases. While the A-D' pre- and posttreatment change may not be as dramatic as the ANB evaluation, the major point to consider is the critical degree of accuracy which this measurement brings into an analysis.