A theater, theatre or playhouse, is a structure where theatrical works or plays are performed, or other performances such as musical concerts may be produced. A theatre used for opera performances is called an opera house. While a theater is not required for performance (as in environmental theater or street theater), a theater serves to define the performance and audience spaces. The facility is traditionally organized to provide support areas for performers, the technical crew and the audience members. There are as many types of theaters as there are types of performance. Theaters may be built specifically for a certain types of productions, they may serve for more general performance needs or they may be adapted or converted for use as a theater. They may range from open-air amphitheaters to ornate, cathedral-like structures to simple, undecorated rooms or black box theaters. Some theaters may have a fixed acting area (in most theaters this is known as the stage), while some theaters, such as black box theaters, may not, allowing the director and designers to construct an acting area suitable for the production.