rod and frame illusion occurs because of the effect of the orientation of the frame on the rod. in the simplest example of the rod and frame illusion, the illusion will cause the participant to perceive the rod to be oriented congruent with the orientation of the frame. when the participant is viewing the rod and frame that are both positioned at 0 degrees (or vertical), he or she perceives the rod as vertical with perfect accuracy. however, when the frame is tilted away from vertical, the participant's perception of vertical is affected. the participant tends to perceive the rod to be tilted in the same direction as the frame is oriented (e.g., if the frame is tilted in the counterclockwise direction, the rod will also be perceived as being tilted counterclockwise). as the tilt of the frame increases, the participants' perceived vertical increasingly deviates from true vertical.