Torque is the rotational force applied to a tooth around its long axis, which requires control over the tooth’s root position. Rectangular wires engage the bracket slot fully, allowing for effective transmission of torque forces due to their geometric shape. In contrast, round wires do not fill the bracket slot and thus cannot deliver sufficient rotational control. Twisted or flexible wires lack the rigidity and dimensional stability necessary for precise torque application. Therefore, rectangular wires are essential for controlled torque movements in orthodontics.

Reference: Proffit, Contemporary Orthodontics, 6th Edition.