The concept of the fourth wall of the theatre stage space that faces the audience is essentially the same. It can be considered as a construct which divides the actors. But since the curtain comes down just behind the proscenium arch, it has a physical reality when the curtain is down. A proscenium stage is different from a thrust stage or an arena stage. Skene is the Greek word for the tent, and later building, at the back of the stage from which actors entered, in the Hellenistic period it became an increasingly large and elaborate stone structure, often with three storeys. In Greek theatre, which unlike Roman included painted scenery, the proskenion might also carry scenery, in the Greek and Roman theatre, no proscenium arch existed, in the modern sense, and the acting space was always fully in the view of the audience. Modern halls designed mainly for orchestral music often adopt similar arrangements, the oldest surviving indoor theatre of the modern era, the Teatro Olimpico in Vicenza, is sometimes incorrectly referred to as the first example of a proscenium theatre. The Teatro Olimpico was a reconstruction of a Roman theatre. It has a plain proscaenium at the front of the stage, dropping to the level, now usually containing stalls seating. However, the Teatro Olimpicos exact replication of the open and accessible Roman stage was the rather than the rule in sixteenth-century theatre design. Engravings suggest that the arch was already in use as early as 1560 at a production in Siena. The earliest true proscenium arch to survive in a permanent theatre is the Teatro Farnese in Parma, Parma has a clearly defined arco scenico—more like a picture frame than an arch, but serving the same purpose—outlining the stage and separating the audience from the action on-stage. While the proscenium arch became an important feature of the traditional European theatre, often becoming very large and elaborate, what the Romans would have called the proscaenium is, in modern theatres with orchestra pits, normally painted black in order that it does not draw attention. In this early modern recreation of a Roman theatre confusion seems to have introduced to the use of the revived term in Italian. There is no evidence at all for this assumption, the Italian word for a scaenae frons is proscenio, a major change from Latin. It would also be possible to retain the classical frons scaenae. The Italian arco scenico has been translated as proscenium arch, the result is that in this theatre the architectural spaces for the audience and the action. Are distinct in treatment yet united by their juxtaposition, no proscenium arch separates them, a proscenium arch creates a window around the scenery and performers. A proscenium theatre layout also simplifies the hiding and obscuring of objects from the audiences view, 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 proscenium arch of the theatre in the Auditorium Building, Chicago. The proscenium arch is the frame decorated with square tiles that forms the vertical rectangle separating the stage (mostly behind the lowered curtain) from the auditorium (the area with seats).
View of the seating area and part of the stage at the Teatro Olimpico (1585) in Vicenza, Italy. No proscenium arch divides the seating area from the "proscenium" (stage), and the space between the two has been made as open as possible, without endangering the structural integrity of the building.
The "proscenium" (stage) at the Teatro Olimpico. The central archway in the scaenae frons (or proscenio) was too small to serve as a proscenium arch in the modern sense, and was in practice always part of the backdrop to the action on-stage.