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Theatre Trivia - Fly System
A fly system, flying system or theatrical rigging system, is a system of lines (e.g. ropes), blocks (pulleys), counterweights and related devices within a theatre that enable a stage crew to quickly, quietly and safely fly (hoist) components such as curtains, lights, scenery, stage effects and, sometimes, people (e.g. Peter Pan). 

Systems are typically designed to fly components between clear view of the audience and out of view, into the large opening, fly loft, above the stage.

Fly systems are often used in conjunction with other theatre systems, such as scenery wagons, stage lifts and stage turntables, to physically manipulate the mise-en-scène.

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).

The line set is the fundamental machine of a typical fly system.

The function of a typical line set is to fly (raise and lower) a slender beam (typically a steel pipe) known as a batten by hoisting it with lift lines (typically synthetic rope or steel cable).

By hanging scenery, lighting, or other equipment to a batten, they in turn may also be flown.

A batten is said to be "flying in" when it is being lowered toward the stage, and "flying out" when it is being raised into the fly space.

Battens may be just a few feet in length or may extend from one wing (side) of the stage to the other. A batten is suspended from above by at least two lift lines, but long battens may require six or more lift lines.

In manual rigging, a line set’s lift lines support weights opposite their connections to the batten in order to balance the gravity load of the batten and whatever it carries.
The lift lines are reeved through a series of pulleys, known as blocks, that are mounted above the stage to fly loft structure.

An operating line (a.k.a. hand line or purchase line) allows riggers on the fly crew to raise and lower the batten.

Automated rigging sometimes uses weights to help balance line set loads in a manner similar to manual counterweight rigging. Otherwise it relies solely on the motor power of an electric hoist to fly a line set.

Line sets are typically general purpose in function, meaning they can perform any number of functions which vary depending upon the requirements of a particular theatre production. For example, a general purpose line set can usually be quickly transformed into a drapery or scenery line set, but converting a general purpose line set into an electrical line set is more involved.

When a line set has a predetermined, relatively permanent, function it is known as a dedicated line set.

Flying rigs are used to fly scenery or performers in a more elaborate fashion than typical line sets.