In the interest of further clarification of the interaction of the multiple causative factors of upper incisor retroclination, the development of Class II Division 2 occlusion is contrasted to that of Class I. The patients whose records were used had not received orthodontic treatment. It was found that upper incisor retroclination in Class II Division 2 occurred after their emergence into the oral cavity. The effect is attributed to the upper incisors being able to erupt well within the cover of the lower lip through a combination of circumstances related to the proportional balance of the patient's facial structures in the vertical dimension. The lower lip would guide the upper incisors into retroclined positions, only if the latter were unobstructed by digit, tongue or other teeth of either arch. This implied not only a critical balance in size and relationship of the facial structures in the vertical dimension but also equally critical balances in the antero-posterior and lateral dimensions, without which the lower lip factor alone would be ineffective.