rail below the arbor, is used to drive a steel cable purchase line. The purchase line is still terminated at the top and bottom of the arbor, but a rope lock is not used on the motor-assist line set. Weight on the arbor helps balance the batten load so that hoist motor size can remain relatively small. It is often feasible to retrofit a standard counterweight line set to become a motor-assist set. Dead-haul systems fly the entire load placed on a line set without the assistance of counterweight. Therefore, dead-haul motor sizes are relatively large. Hoist (winch) motors are either fixed speed or variable speed. Fixed speed motors are used at heavy-load and/or slow-speed line sets (e.g., electrics and orchestra shell line sets). Variable speed motors are used at line sets requiring dynamic motion that may be viewed by the audience (e.g., drapery and scenery line sets). Scenery hoists commonly allow travel at rates of hundreds of feet per minute. Digital control systems incorporating computers or programmable logic controllers (PLCs) have become commonplace as well, bringing their advantages of high accuracy, safety and repeatability to fly systems. Components Battens Battens are linear members to which live loads may be attached for flying. Battens were made of wood originally, but today they are typically steel pipe. Loads mounted to battens include lights, curtains and scenery so they may travel vertically, be raised up into the fly space (flown out) or lowered near to the stage floor (flown in) by its associated line set. Battens typically stretch the width of the stage, parallel with the proscenium wall, and are maintained level (parallel to the stage deck) regardless of elevation. When a batten is flown all the way out (close to the grid) it is at high trim. When it is flown all the way in (usually to about above the stage deck) it is at low trim. Loads are attached to the batten in various ways. Most lighting fixtures, for example, utilize a C-clamp to rigidly secure the light onto the batten, in conjunction with a safety cable that is looped around the batten to prevent the light from falling should the C-clamp connection fail. Non-traveling curtains (e.g., borders) often employ cloth ties, similar to shoestrings, that are hand tied onto the batten. Battens are suspended by evenly spaced lift lines, with pick points generally apart. The unsupported, cantilevered, ends of