What is an proscenium arch stage?

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 main stage is the space behind the proscenium arch, often marked by a curtain which 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.