Muppet Theater Stage

From Muppet Wiki

The Muppet Theater Stage is the area contained by the proscenium of the Muppet Theater. The stage is where most of the action on The Muppet Show takes place, and its loft is home to a number of flats for set pieces. The back wall (seen whenever there isn't a curtain or set in the way) is made of brown bricks. There are also trap doors on the stage (as revealed in episodes 118 and 517).