A proscenium (Greek: προσκήνιον, proskḗnion) is the metaphorical vertical plane of space in a theatre, usually surrounded on the top and sides by a physical proscenium arch (whether or not truly "arched") and on the bottom by the stage floor itself, which serves as the frame into which the audience observes from a more or less unified angle the events taking place upon the stage during a theatrical performance. The concept of the fourth wall of the theatre stage space that faces the audience is essentially the same.
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It can be considered as a social construct which divides the actors and their stage-world from the audience which has come to witness it. But since the curtain usually comes down just behind the proscenium arch, it has a physical reality when the curtain is down, hiding the stage from view. The same plane also includes the drop, in traditional theatres of modern times, from the stage level to the "stalls" level of the audience, which was the original meaning of the proscaenium in Roman theatres, where this mini-facade was given more architectural emphasis than is the case in modern theatres. A proscenium stage is structurally different from a thrust stage or an arena stage, as explained below.