A proscenium theater is a specific style of theater. Several features define a proscenium theater, and this particular theater layout is extremely common; if you have ever been to see a live performance, especially in a high school auditorium, chances are high that you have seen a proscenium theater. In addition to proscenium style theaters, it is also possible to find black box theaters, theaters with thrust stages, theaters in the round, and numerous other configurations of stage and audience.
The classically defining feature of a proscenium theater is the proscenium, an arch which frames the stage for the audience. In addition, the audience faces the stage directly, with no audience on the sides of the stage, and the stage in a proscenium theater is typically raised, allowing the audience to see more clearly. Modern proscenium theaters sometimes lack the proscenium, but they are still called “proscenium theaters” because they retain the other characteristics of this style of theater.
Proscenium theaters originated in the 1600s, and became immensely popular by the 1700s. There are certain advantages of a proscenium theater, such as the fact that the stage doesn't have to be as open, allowing people to conceal props, sets, and orchestras in the wings or near the stage without having these things visible to the audience. A proscenium theater also creates a sense of staged grandeur, with the proscenium arch acting almost like a picture frame, giving the audience the sense that they are looking into a scene.
Classically, proscenium theaters have curtains which are used to conceal the stage during set changes and intermissions, rising up behind the arch to reveal the stage. The area in front of the curtain, which is visible at all times, is known as the apron, and some performances may be played entirely on the apron. Behind the proscenium arch are the wings, the areas adjacent to the stage which cannot be seen by the audience, with steps which allow actors and crew to access the stage.
Some actors and crew find the proscenium design very limiting. The wider angle of the thrust stage, where the audience surrounds a section of the stage, allows for more natural staging, because actors do not need to play to a specific location for the audience to see them. Black box theaters offer more flexibility than proscenium theaters, along with a more intimate setting, while theaters in the round, where the audience surrounds the entire stage, present interesting and sometimes fun challenges and situations.