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What is 'Torque' in orthodontics refers to: and explain in detail?
Clinical heights of the crowns, contact areas, their tip (mesio distal angulations) and torque (labiolingual inclination) contribute to the aesthetics of the smile. 
Therefore torque refer to the labiolingual inclination of the teeth.
Reference: Orthodontics: Diagnosis and Management of Malocclusion and Dentofacial Deformities 3rd O P Kharbanda, pdf no 789