In Angle's classification, the key to determining occlusion is the position of the upper first permanent molar because it serves as a stable landmark for the anteroposterior relationship of the dental arches. The mesiobuccal cusp of the upper first molar occludes with the buccal groove of the lower first molar, defining Class I, II, or III occlusal relationships. This allows for a standardized assessment of malocclusion and guides orthodontic diagnosis and treatment. The canines and other teeth are considered secondary references in this system.

Reference: Textbook of Orthodontics, 2nd Edition.