The Ames trapezoid (or Ames window) is an image on, for example, a flat piece of cardboard, in the perceived shape of a style of window. When observed it appears to the human eye as a rectangular window but is, in fact, a trapezoid.
Both sides of the piece of cardboard have the same image. The cardboard is hung vertically from a wire so it can rotate around continuously, or is attached to a vertical mechanically rotating axis for continuous rotation.
When observed the window appears to rotate through less than 180 degrees. The exact amount of travel that is perceived varies with the dimensions of the trapezoid. It seems the rotation stops momentarily and reverses its direction. It is therefore not perceived to be rotating continuously in one direction but instead is mis-perceived to be oscillating.
See also: Ames Room
This phenomenon was discovered by Adelbert Ames, Jr..