then down, terminating at the batten, a load-bearing pipe that spans much of the width of the stage. If the loft blocks are mounted to the grid deck, at the loft block wells, the system is called a grid-mount, or upright counterweight rigging system. If the loft blocks are mounted to roof beams (loft block beams), the system is called an under-hung counterweight rigging system. Under-hung systems have the advantages of maintaining a clear grid deck surface for spot rigging and facilitating crew movement across the grid. The arbor's vertical position is controlled by means of a rope known as the operating line, hand line, or purchase line. The operating line forms a loop by running from the bottom of the arbor down to and around the tension block, through the rope lock, up and over the head block and back down (alongside the lift lines), where it terminates at the top of the arbor. The head and tension blocks are located above and below the full extent of the arbor's travel (movement), respectively, thereby enabling an operator to pull the operating line up or down to move the arbor. When the arbor is raised via the operating line, the lift lines slacken, which causes the batten to lower under its own weight (and the weight of its load, if any). Conversely, when the arbor is lowered, the lift lines increase in tension, which in turn causes the batten to rise. The combined weight of the arbor and its counterweights initially matches that of the batten so that when the batten is not being raised or lowered, it will tend to remain motionless at any arbitrary elevation above the stage. As more weight is added to the batten (in the form of curtains, scenery, lighting equipment, and rigging hardware), the system is rebalanced by adding more counterweights to the arbor. When the system is properly balanced, an unassisted operator (flyman) can lift the batten and its arbitrarily heavy load from the stage ("fly it out", in theatrical jargon), completely above the proscenium and out of view of the house, sometimes to heights in excess of . Some large theatres, such as the Metropolitan Opera House (Lincoln Center), have more than 100 independent, parallel counterweight line sets, while smaller venues may only have a few line sets for the most frequently adjusted loads, such as electrics. Double-purchase counterweight system Double purchase