What is a double purchase fly system? Simply, its a way to move set pieces and curtains vertically on a stage.
A more complex description goes like this: The fly system operates on a system of weights and pulleys. In the Fogelberg Auditorium, there are 16 battens (or long metal pipes) to which curtains, backdrops, cycs, even set pieces can be affixed. Each batten is attached to a pulley with a balancing weight on the arbor. A double-purchase fly system has two pulleys for each batten. This means that the length of the rope is halved, so the "fly box" where the ropes are operated to fly the pipes is actually a number of feet above the "deck" (or stage floor). This also means that there is a 2:1 weight ratio for the arbor. That is, for every 100 pounds on the batten, 200 pounds is necessary on the arbor for the pipe to be balanced and not come crashing down or flying up recklessly into the flyspace. A balanced pipe can be controlled to fly in or out for scene changes in a performance, or to set curtains, legs, and valances to their proper heights or settings.
Still confused? Follow this link to a wikipedia article about fly systems: http://en.wikipedia.org/wiki/Fly_system