In theatrical stage rigging
the purchase line
is the rope which controls the counterweight fly system
. The rope attaches at the top of the arbor
, runs up through the head block
, back down to the stage
floor and through the bottom idler
. It is tied off again at the bottom of the arbor.
Pulling on either the front or back purchase line controls the location of the arbor on the T-bar track, and thus moves the attached wire ropes, which controls the height of the batten.
Pulling down on the front purchase line raises the arbor and lowers the batten. Likewise, when pulling on the back purchase line the scenery will raise as the arbor is lowered.
Many larger counterweight fly systems are double purchase systems. This means that for every distance the arbor is moved by the purchase line, the batten will move twice that distance. This allows for faster scene shifts. The trade-off in a double purchase system is that in order to balance the load, the amount of counterweight on the arbor must be twice the weight of the load on the batten.
Part of the Stage Rigging Metanode