This stage at the Lexington Opera House is a proscenium, one of the main types of stages. The key feature of this type of stage is an arch or frame, like a huge picture frame, through which the audience watches the action. This is called the proscenium arch. The proscenium curtain, sometimes called the house curtain, covers the proscenium opening. The curtain is raised or opened at the beginning of the play and lowered or closed to signify the end of scenes, acts, or the play itself.
(FYI: The term proscenium comes from proskenion, a flat space behind the chorus in the hillside amphitheaters of ancient Greece. Eventually, the flat playing area was elevated, as most proscenium stages are today. In the theaters of ancient Rome, a curtain was actually held up in front of the playing area, then dropped and carried away by slaves at the beginning of a play. The curtain was first raised, rather than dropped, in the 1500s and 1600s in Renaissance Italy.)