http://th07.deviantart.net/fs71/200H/f/2012/009/3/7/the_third_wall_by_thehanyoualchemist-d4lv3xi.jpg

http://en.wikipedia.org/wiki/Fourth_wall

THIRD WALL: Usually referred to as the "fourth wall," depending upon how a stagebuilder numbers the sides of the stage, the third or fourth wall is an imaginary barrier that separates the events on stage from the audience. The idea is that the stage background is constructed with a cutaway view of the house, so that the people sitting on the audience can look through this invisible "fourth wall" and look directly into the events inside. Such stages preclude theater-in-the-round and they require a modified apron stage with an expensive reproduction of an entire house or building, often complete with stairs, wallpaper, furniture, and other bits to add verisimilitude. This type of stage became increasingly common within the last two centuries, but the money involved in constructing such stages often precludes their use in drama, leaving arena stages most popular for the architectural design of the stage.