The meshing of a gear tooth begins as a sliding motion on the dedendum, transitions to rolling motion at the pitch circle, and ends as the sliding motion again on the addendum. Understanding the interaction of gear teeth is essential to identifying correct verse unusual wear on gear teeth. Gears can wear improperly due to improper operation because of some other defect that causes the gears to mesh differently than the designer intended. Poorly meshing gear teeth can also be due to a cause outside of the gearbox.