Ames trapezoid

The Ames trapezoid or Ames window is a style of window which, when observed frontally, appears to be a rectangular window but is, in fact, a trapezoid.

The window is mounted on a rod connected to an electric motor that rotates it about its vertical axis. When it is observed with one eye from about 3 meters or with both eyes at 6 meters, or more, the window appears to rotate through 180 degrees and then seems to stop momentarily and reverse its direction of rotation. It is therefore not perceived vertically to be rotating continuously in one direction but instead is mis-perceived to be oscillating, reversing its direction once every 180 degrees.