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See also Fly Tower. The area above the stage where scenic pieces are stored out of sight. Often further delineated by the maximum trim height of a batten: A theatre with Full Flyspace will have a grid two to two-and-a-half times the height of the proscenium or typical trim of the Grand Valance. A theatre with Half Flyspace will be one-and-a-half times as high as the proscenium. A theatre with No (or Zero) Flyspace may have battens and a counterweight system, but the maximum high trim will be just slightly above that of the proscenium (battens can fly in but not out, above normal playing trim).
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The 2 1/2 times rule drives me nuts. It was once an appropriate rule of thumb - decide how high a proscenium opening should be for function and aesthetics and assume the high trim should be 2 1/2 to 3 times as high. Now someone decides on a stage roof height on a basis that has nothing to do with what will be on stage, probably budget, and then sizes the proscenium using the 2 1/2 rule - and that doesn't account for the difference between high trim and roof height - usually 4 to 5 feet. Another stupid piece of lore that allows someone who really doesn't understand what goes on in a stage to proclaim themselves an expert and argue for another bad building. There are so many 12 and 15 foot prosceniums out there because of this mistaken and misused factor, it's criminal. If your are planning for band or orchestra or chorus, you need to be in the low to mid twenty feet range minimum. Period. Then determine what high trim you can live with for drama use. For me it has to do with orchestra shell towers and the basic masking setting and being able to move those towers without catching the masking. This also worked when the “golden rectangle” rule of width to height of proscenium was applied. 48' wide opening gets a 30' high opening using this pleasing aspect ration, which would result in a respectable 75' high trim. But clearly the misapplication of the 2 ½ times rule that results in 60' wide by 16' high letterbox prosceniums misses the boat. Many other considerations but this is a start.

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