Dental casts are mounted to dental articulators using anatomical records in order to simulate and duplicate static and dynamic interocclusal relationships. The three dimensional relationship of the maxillary arch to the transverse horizontal axis of the mandible is defined by establishing a third point of reference. The purpose of this study was to examine the relationship of the Frankfort horizontal plane and the HIP plane to the horizontal when the patient is in the natural head position. Ten subjects were evaluated in the natural head position with angular measurements made from lateral photographs and lateral skull radiographs to determine the relationship of these two reference planes to the true horizontal plane. The mean, standard deviation and coefficient variation were calculated for both reference planes. The clinical significance of this information is to verify or modify those techniques used for mounting dental casts to the dental articulator, which is used in treatment planning and laboratory procedures