Consider a cylinder shown in figure below.
The torque is applied at the top plane of the cylinder. The bottom of the cylinder (passing through the point ) is fixed. Consequently, the applied torque creates deformation. Consider two points and at the top. The applied torque will cause displacement of point to point . The described angle is called the angle of twist. On the other hand, the angle is called shear strain.
The stress distribution is shown in figure 2 below.