Accuracy of reproducing angles: is a right angle special?

The ability to set the angle of a 'V' to a designated value in the following three conditions was compared: (1) verbal designation of V angle; (2) initial 30 s visual demonstration of the designated V angle; (3) verbal designation of V angle plus feedback after every setting. The designated angles were 90 degrees and 45 degrees plus three arbitrary angles (65 degrees, 125 degrees, and 145 degrees). Each run comprised thirty consecutive settings. To ensure that our observers based their settings entirely on V angle it was arranged that line orientation did not provide a reliable cue to V angle. In condition (1), accuracy of setting V angle was significantly worse when the designated angle was other than 90 degrees or 45 degrees. This was not the case in condition (2), indicating that observers maintained a memory of the initial demonstration throughout a run of thirty settings. Setting error was not significant in condition (3) for any of the five angles. However, even in condition (3), setting-to-setting variability was significantly lower for the 90 degrees angle than for the other angles.