elevator stage

elevator stage, drop stage, lift stage

A theater stage floor which moves vertically on an elevator, usually so that one set can quickly replace another; may consist of a single unit or articulated sections.
McGraw-Hill Dictionary of Architecture and Construction. Copyright © 2003 by McGraw-Hill Companies, Inc.
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We built a space in 1961 that seated 350 and had a big elevator stage that went down from first floor to basement--and up would go the new set, because you can't change sets in the round.