In this way, the fourth wall exists regardless of the presence of any actual walls in the set, the physical arrangement of the theatre building or performance space, or the actors' distance from or proximity to the audience. The actors ignore the audience, focus their attention exclusively on the dramatic world, and remain absorbed in its fiction, in a state that the theatre practitioner Konstantin Stanislavski called "public solitude" (the ability to behave as one would in private, despite, in actuality, being watched intently while so doing, or to be 'alone in public'). The fourth wall, though, is a theatrical convention, rather than of set design. When a scene is set indoors and three of the walls of its room are presented onstage, in what is known as a box set, the fourth of them would run along the line (technically called the proscenium) dividing the room from the auditorium. The metaphor suggests a relationship to the mise-en-scène behind a proscenium arch.
From the 16th century onward, the rise of illusionism in staging practices, which culminated in the realism and naturalism of the theatre of the 19th century, led to the development of the fourth wall concept. While the audience can see through this wall, the convention assumes the actors act as if they cannot.
The fourth wall is a performance convention in which an invisible, imaginary wall separates actors from the audience. It is the frame decorated with square tiles that forms the vertical rectangle separating the stage ( mostly behind the lowered curtain) from the auditorium ( the area with seats). The proscenium arch of the theatre in the Auditorium Building, Chicago.