clear openings through which the lift lines of under-hung counterweight, or automated, systems may pass. A grid deck is indispensable in professional and touring theatres, and desirable in all theatres with a fly tower, providing invaluable access and flexibility to fly systems. However, due to height limitations, not all fly towers are equipped with a grid. Transverse catwalks are sometimes provided as compensation for the lack of a grid. San Francisco's War Memorial Opera House, not burdened by height limitations, has two grid decks. Loading bridge Specific to a stage house using a counterweight system, the loading bridge, or loading gallery, is a catwalk vertically positioned below the headlock beam, and above the fly gallery. The loading bridge is used to add or remove counterweights from arbors. The floor of the loading bridge is also typically used as a storage area for uncommitted counterweights that are available for loading onto counterweight arbors. Stage houses with especially tall fly towers, or double-purchase systems, may have two loading bridges, one stacked over the other to facilitate the loading of relatively tall arbors. Fly gallery A fly gallery is a catwalk running from the proscenium wall to upstage wall to which a pinrail and/or locking rail may be mounted used by the fly crew to operate the fly system. The fly gallery elevation is typically at about proscenium height, providing a good view of the stage and fly loft. Fly galleries may be provided stage left and right, or at just one side. Where provided at both sides of the stage they may be connected by a cross-over catwalk at the upstage wall. It is possible to load arbors (add or remove counterweights) at the fly gallery, but standard practice is to load arbors at the loading bridge. (A vertical cinematic montage from the deck to the fly gallery is a striking feature of Orson Welles' Citizen Kane.) Pin rail A pin rail, originally a wood beam, is typically a large-diameter round steel tube with vertical through-holes that accept belaying pins used in a hemp rigging system. Depending on the pin rail design, the pins may be removable or permanently fixed to the rail. Pin rails are typically installed permanently at the onstage edge of the fly gallery(ies), extending from the proscenium wall to upstage wall, sometimes in a stacked (rail over rail) arrangement. Mobile pin rails may also be used and are