The fourth wall is the imaginary "wall" at the front of the stage in a traditional three-walled box set in a proscenium theatre, through which the audience sees the action in the world of the play. The idea of the fourth wall was made explicit by philosopher and critic Denis Diderot and spread in 19th-century theatre with the advent of theatrical realism, which extended the idea to the imaginary boundary between any fictional work and its audience. Speaking directly to or otherwise acknowledging the audience through the camera in a film or television program, or through this imaginary wall in a play, is referred to as "breaking the fourth wall" and is considered a technique of metafiction, as it penetrates the boundaries normally set up by works of fiction.
This should not be confused with the aside or the soliloquy, dramatic devices often used by playwrights where the character on stage is delivering an inner monologue, giving the audience insight into his or her thoughts. The fourth wall is also used for comedic purposes.
Convention of modern theatre 
The presence of the fourth wall is an established convention of modern realistic theatre, which has led some artists to draw direct attention to it for dramatic or comedic effect when this boundary is "broken", for example by an actor onstage speaking to the audience directly. This is common in children's theatre where, for example, a character might ask the children for help.
The acceptance of the transparency of the fourth wall is part of the suspension of disbelief between a fictional work and an audience, allowing them to enjoy the fiction as if they were observing real events. Although the critic Vincent Canby described it in 1987 as "that invisible screen that forever separates the audience from the stage," postmodern art forms frequently either do away with it entirely, or make use of various framing devices to manipulate it to emphasize or de-emphasize certain aspects of the production.
Outside theatre 
Lowry ... stood across the road from his subjects and observed. Often enough there are a number of individuals in a crowd peering back at him. They invite us momentarily into their world, like characters on a stage sometimes do, breaking the fourth-wall illusion.
McKellen justifies this application of the theatre term to Lowry's art by explaining that "Lowry’s mid-air viewpoint is like a view from the dress circle", looking down as if to a stage. And, McKellen argues, Lowry "often marks the limits of the street scene with curbstones or a pavement that feel like the edge of the stage where the footlights illuminate the action."
The metaphor of the fourth wall has been applied by literary critic David Barnett to The Harvard Lampoon's parody of The Lord of the Rings when a character breaks the conventions of storytelling by referring to the text itself. The character Frito observes "it was going to be a long epic", which in Barnett's view "breaks the 'fourth wall'".
On Television, breaking the fourth wall is rare, though a regular proponent of it is the main character of British Sitcom Mrs Brown's Boys, Mrs Brown, regularly breaks the fourth wall, talking to the audience, and will run from set to set (for instance to retrieve a prop needed from a scene, left in another set). Mrs Brown will also drop character to comment on a situation. 
Fifth wall 
The term "fifth wall" has been used as an extension of the fourth wall concept to refer to the "invisible wall between critics or readers and theatre practitioners." This conception led to a series of workshops at the Globe Theatre in 2004 designed to help break the fifth wall. The term has also been used to refer to "that semi-porous membrane that stands between individual audience members during a shared experience." In media, the television set has been described metaphorically as a fifth wall because of how it allows a person to see beyond the traditional four walls of a room. In shadow theatre the term "fifth wall" has been used to describe the screen onto which images are projected.
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