Blocks are either upright, when mounted atop a support structure, or under-hung, when mounted to the underside of a support structure. The side plates of blocks preferably fully cover the profile of (fully enclose) the sheaves to lend the block greater stability and limit the sheave's (and crew's) potential for damage from foreign objects. Nevertheless, blocks are available with exposed sheaves. Loft block A Loft block is an overhead block that supports a single lift line. A loft block supports and redirects a lift line from the batten to the head block of a line set. Under-hung loft blocks typically mount to loft block beams (fly loft roof beams). Upright loft blocks typically mount to loft block wells (grid-level structural channels). A spot block is a readily movable loft block for mounting anywhere on the grid deck for spot rigging. The diameter of a loft block sheave for galvanized aircraft cable is typically at least 32 times the diameter of the cable. For example, loft blocks are typically used with GAC, but blocks may be used to facilitate flying heavier line sets (e.g., electrics). Loft blocks may be equipped with idler pulleys or sag bars to limit the sag of horizontally running lift lines on under-hung systems. In under-hung counterweight systems that use upright head blocks the series of loft blocks immediately following the head blocks are typically multi-line loft blocks instead of single-line to account for built-in vertical misalignment between head blocks and loft blocks. Head block Head blocks are overhead multi-line blocks used for the lift lines and operating lines. Head blocks support and redirect all the lift lines from loft blocks to sand bags (of a hemp set), counterweight arbor (of a counterweight set) or hoist (of an automated line set). Rope line (hemp) head blocks are typically upright blocks that mount to the rope line head block well channels at the grid level. In a counterweight rigging system the head block sheave is grooved for both the steel cable lift lines and an operating line, with the groove for the operating line provided at the middle of the multi-grooved sheave, between the lift lines. Counterweight head blocks mount atop or at the underside of the head block beam, depending on the beam's vertical position. The diameter of a head block sheave used for galvanized aircraft cable is typically at least 48 times the diameter of the