The fourth wall is a common convention in narrative drama in which a metaphorical, invisible, or imaginary wall separates performers from the audience, so that the audience sees through this "wall" into the performed narrative, but the performers behave as if they cannot see the audience in turn. The metaphor has also been extended outside of the theater, for instance to the typical boundary between character and audience in films, videos, or television programs, in which characters behave as if they are unaware of the camera. From the 16th century onward, the rise of illusionism in staging practices—culminating in the realism and naturalism of the theatre of the 19th century—led to the development of the fourth wall concept.