A Proscenium theatre is a theatre space whose primary feature is a large frame or arch (called the proscenium arch even though it is frequently not a rounded archway at all), which is located at or near the front of the stage. The use of the term "proscenium arch" is explained by the fact that in Latin, the stage is known as the "proscenium", meaning "in front of the scenery."
In a proscenium theatre, the audience directly faces the stage, which is typically raised several feet above front row audience level, and views the performance through the proscenium "arch". The space that holds the audience is called the "house". The main stage is the space behind the proscenium arch; it is often delimited by a front curtain that can be lowered or drawn closed. The space in front of the curtain is called the "apron". The stage-level areas obscured by the proscenium arch and any curtains serving the same purpose (often called legs or tormentors) are called the wings, while the space above the stage that is concealed by the top of the proscenium arch is called the flyspace. Any space not viewable to the audiences is collectively referred to as offstage. Proscenium stages range in size from small enclosures to several stories tall.
In general practice, a theatre space is referred to as a "proscenium" any time the audience directly faces the stage, with no audience on any other side, even if there is not a formal proscenium arch over the stage. Because it seems somewhat incongruous to refer to a proscenium theatre when no proscenium arch is present, these theatres are sometimes referred to as "end-on" theatre spaces.
In ancient Rome, the stage area in front of the scaenae frons was known as the "proscenium", meaning "in front of the scenery". In the Roman theater, no proscenium arch existed, in the modern sense. However, Roman theaters were similar to modern proscenium theaters in the sense that the entire audience had a restricted range of views on the stage--all of which were from the front, rather than the sides or back.
The oldest surviving indoor theater of the modern era, the Teatro Olimpico in Vicenza (1585), is sometimes incorrectly proposed as the first example of a proscenium theatre. The Teatro Olimpico was an academic reconstruction of an outdoor Roman theater. This emulation of the Roman model extended to referring to the stage area as the "proscenium", and some writers have incorrectly referred to the theater's scaenae frons as a proscenium, and have even suggested that the central archway in the middle of the scaenae frons was the inspiration for the later development of the full-size proscenium arch. There is no evidence at all for this assumption (indeed, contemporary illustrations of performances at the Teatro Olimpico clearly show that the action took place in front of the scaenae frons, and that the actors were rarely framed by the central archway).
The confusion is probably the result of the fact that the Italian word for a scaenae frons is "proscenio." One modern translator explains the wording problem: "[In this translation from Italian,] we retain the Italian proscenio in the text; it cannot be rendered proscenium for obvious reasons; and there is no English equivalent....It would also be possible to retain the classical frons scaenae. The Italian "arco scenico" has been translated as "proscenium arch."
In practice however, the stage in the Teatro Olimpico runs from one edge of the seating area to the other, and only a very limited framing effect is created by the coffered ceiling over the stage and by the partition walls at the corners of the stage where the seating area abuts the floorboards. The result is that in this theater "the architectural spaces for the audience and the action . . . are distinct in treatment yet united by their juxtaposition; no proscenium arch separates them."
However, the Teatro Olimpico's exact replication of the open and accessible Roman stage was the exception rather than the rule in sixteenth-century theatre design. Engravings suggest that the proscenium arch was already in use as early as 1560 at a production in Siena. The most likely candidate for the first true proscenium arch in a permanent theatre is the Teatro Farnese in Parma (1618). A clearly defined "arco scenico"--more like a picture frame than an arch, but serving the same purpose--outlines the stage and separates the audience from the action onstage.
The proscenium arch creates a "window" around the scenery and performers. The advantages are that it gives everyone in the audience a good view because the performers need only focus on one direction rather than continually moving around the stage to give a good view from all sides. A proscenium theatre layout also simplifies the hiding and obscuring of objects from the audience's view (sets, performers not currently performing, and theatre technology). Anything that is not meant to be seen is simply placed outside the "window" created by the proscenium arch, either in the wings or in the flyspace above the stage.
The side of the stage that faces the audience is referred to as the "fourth wall". The phrase "breaking the proscenium" refers to when a performer addresses the audience directly as part of the dramatic production (is also known as breaking the fourth wall). The phrase can also refer to when a member of the cast or crew walks onto the stage or into the house when there is an audience inside, also breaking the fourth wall.
Proscenium theatres have fallen out of favor in some theatre circles because they perpetuate the fourth wall concept. The staging in proscenium theatres often implies that the characters performing on stage are doing so in a four-walled environment, with the "wall" facing the audience being invisible. Many modern theatres attempt to do away with the fourth wall concept and so are instead designed with a thrust stage that projects out of the proscenium arch and "reaches" into the audience (technically, this can still be referred to as a proscenium theatre because it still contains a proscenium arch, however the term thrust stage is more specific and more widely used).