The basic trick involves a stage that is specially arranged into two rooms, one that people can see into or the stage as a whole, and a second that is hidden to the side, the "blue room". A plate of glass (or Plexiglas or plastic film) is placed somewhere in the main room at an angle that reflects the view of the blue room towards the audience. Generally this is arranged with the blue room to one side of the stage, and the plate on the stage rotated around its vertical axis at 45 degrees. Care must be taken to make the glass as invisible as possible, normally hiding the lower edge in patterning on the floor and ensuring lights do not reflect off it.