In Denavit–Hartenberg parameters, what does theta_i represent?

• A. The link length between frames.
• B. The twist angle between z axes.
• C. The joint angle around the z axis of the previous frame.
• D. The offset along the previous z axis.

Answer: (C)

The angle theta_i is the rotation about the z axis of the previous frame that moves the orientation from frame i-1 to frame i. In the standard DH procedure, you rotate around z_{i-1} by theta_i to align x_{i-1} with x_i, then proceed with the other translations and rotations. This value is the joint variable for revolute joints, meaning it represents the actual rotational position; for prismatic joints, theta_i stays fixed and the motion is captured by d_i along z_{i-1}. The other DH parameters correspond to link length (a_i), the twist between z_{i-1} and z_i (alpha_i), and the offset along z_{i-1} (d_i).