I'm reading Theory of Applied Robotics, 2nd edition by Jazar. The author states that articulated configuration has the following form: $R \vdash R \perp R$ where $R$ stands for revolute joint, $\vdash$ represents that 2 adjacent joint axes are orthogonal (intersect at right angle) and $\perp$ represents that 2 adjacent joint axes are perpendicular (they close right angle with common normal). The example of articulated configuration is given in the image
So by looking at the image axes $z_1$ and $z_2$ are supposed to be perpendicular but they look parallel to me. What am I missing ?