Cooperman  and Rich  observed in their study that the occlusal plane showed a close relationship to the hamular notch–incisive papilla plane. Fu et al.  in their study used a three dimensional surveying software to check parallelism between the hamular notch–incisive papilla plane and four types of occlusal planes formed by different anterior and posterior reference points on maxillary teeth and found out that parallelism exists between the natural occlusal plane (defined as mesial-incisal edge of upper right central incisor and distal-buccal cusp tips of upper first molars) and hamular notch–incisive papilla plane. Jayachandran et al.  evaluated the reliability of the hamular notch/incisive papilla plane (HIP) in establishing the occlusal plane. The study was done both in dentulous as well as edentulous patients. In dentate subjects, the maxillary stone cast was mounted on the Wills surveyor with HIP made parallel to the horizontal plane using the tripoding method. The vertical distance between the occlusal plane and floor of the surveyor was measured at four points. When the measured values were equal, the two planes were confirmed to be parallel for that situation. In the edentulous subjects, the occlusal plane, established clinically using the ala-tragal line, was compared with the HIP radiographically using lateral cephalograms. They concluded that the HIP was parallel to the occlusal plane.