A fly system, or theatrical rigging system, is a system of rope lines, blocks (pulleys), counterweights and related devices within a theater that enables a stage crew to fly (hoist) quickly, quietly and safely components such as curtains, lights, scenery, stage effects and, sometimes, people. Systems are typically designed to fly components between clear view of the audience and out of view, into the large opening, known as the fly loft, above the stage.
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Theatrical rigging is most prevalent in proscenium theatres with stage houses designed specifically to handle the significant dead and live loads associated with fly systems. Building, occupational safety, and fire codes limit the types and quantity of rigging permitted in a theatre based on stage configuration. Theatrical rigging standards are developed and maintained by organizations such as USITT and ESTA (now PLASA).