Fusing the first stereo pair (either way) should make the nail either stick out or be behind the window. The second stereo pair nail will have the opposite depth relative to the first stereo pair. Placing both nails in the same window, one would expect one nail to protrude and the other to recede, but due to lack of other cues, stereopsis fails us in this case! For more examples try, Double Nail illusion with motion Hammering nail illusion Two different anaglyph versions are provided below, for those that can't freeview or don't have the proper viewing aids for stereopairs. Back to Gabriel's 3-D Notebook

Double-Nail Illusion