bolted down to the stage deck where needed. Locking rail A locking rail is typically a steel angle or rectangular tube to which the rope locks of a counterweight system are mounted. Locking rails are located on the stage deck and/or fly gallery and typically extend from the proscenium wall to the upstage wall. A stage-level locking rail may be provided with an engaging bar for a portable capstan winch. Arbor pit Arbor pits, where provided, are troughs at the stage edge that provide additional vertical travel to a counterweight system's arbors. Providing a counterweight arbor pit can help compensate for height limitations of a fly tower. The trough depth typically ranges from 2 to 10 feet. Shallower pits may be accessible only from above at the stage deck. Deeper pits are sometimes accessible from a trap room or orchestra pit. Operation Because fly systems involve large amounts of weight, and particularly because the weight is usually suspended above people, there are a number of common precautions taken to ensure safety and prevent injuries. Communication, inspection, and loading procedure are key to the safe operation of a fly system. Calling movement Except for during performances and some rehearsals, a standard practice in theatre is for the flyman to always call (shout) out a warning before moving a lineset so as to alert personnel (e.g., rehearsing performers and technicians) who are on the stage. People on stage typically acknowledge the operator's warning by yelling out a confirmation that the warning was heard. The flyman's warning specifies what is moving and its direction of movement. For example, a particularly verbose call might be something like "lineset three, first electric flying in to the deck, downstage" (in USA) or "Heads onstage, Bar 3, LX 1 coming in." (in UK). In many theaters, all people on stage are expected to respond with "thank you." Upon completion of the lineset motion, some operators may call again (e.g., "lineset three locked") to announce that the lineset has stopped moving. Unbalanced loads Unbalanced loads are of great concern in manual rigging. Minor imbalance is sometimes desirable, for example so that as an operating line is let out a line set will fly in of its own accord. However, as it is common for many thousands of pounds of equipment and scenery to be flown above cast and crew, major imbalance is a grave hazard, and, if left